Alright, so back to our subject of filters. I will try to clarify things further. It's long so read it only if you have an interest to have better understanding of the industry. We will study RO system today, but hold on! I'm not saying RO is a must for everyone, but we will use RO as an "golden standard" if you'd like.
Intro

Water and water treatment industry is one of the most corrupt industries in the world. I'm not talking only about filters now, but also about bottled water, soft drinks as well as gimmicks like "ionizers", "live water" etc. It's an extremely profitable industry where interests of many huge corporations, governments, small players and even world organizations intersect...

Now, please get me right. I'm not a bloody communist who doesn't believe in making profit. Any business must make healthy and HONEST profits, so there's nothing wrong with that. What I find unacceptable is when a company or whole industry represents something old and simple as "super-nano-ultra-mega" advancement and is able to make 5000% profits from the lack of knowledge from ordinary people. I stress the words ORDINARY PEOPLE because it doesn't work in industrial applications. Industry is able to hire people like me, who spent years studying water treatment, so when a salesman from another "snake oil" company comes to us, he's immediately kicked out. As an ordinary consumer, you don't have this luxury, so unless you have some basic knowledge, it's easy to become a pray to a salesman. I will try my best to help you, without selling anything, but to quickly teach you how to filter BS from the truth.
Reverse Osmosis

Before you read further, please forget EVERYTHING you heard about "dead water", "we get minerals from the water" and all other crap. This is all BS and politics. At some point, some very powerful companies started losing too much money because of RO systems being implemented everywhere and public started to switch from buying SAME RO water in plastic bottles to reusable bottles, filling these up for free! Internet got flooded with "RO is bad for you" articles. They even influenced WHO, and WHO released few articles based on old Soviet articles from 1980s that were based on older research of industrial distillation systems in USSR (can check it on WHO website, I'm not making it up).

Quality RO system produces the cleanest, best drinking water, period (ok, there is also distillation, but let's ignore it for now, since it's not easily available consumer system).

But RO system does waste a lot of water. It's also very slow (50-100ml per minute), sensitive to water pressure, requires a lot of maintenance and usually has a tank... Now this tank is the weakest point of the system. This is where bacteria grow, this is where your water is sitting inside a plastic bag... no so good. There are tankless systems out there, but unless you're ok with 50-100ml/minute and waiting 15 minutes to fill your kettle, these systems are more expensive. In short, unless your water has way too many dissolved minerals, you were told by doctors to drink RO water,  you're paranoid or you have special use for this system (ultrasonic humidifiers for example) - RO is a bit of overkill (my personal opinion).

But why do I even talk about RO then? Because understanding RO will make you understand any other system out there. Because of the nature of RO system, you can not play around with "nano-ultra-super", you play with it, and your RO membrane goes bust in no time. By comparing ANY system to RO, you can understand exactly what are you giving up and whether the system makes sense and whether it is overpriced.

So RO system is always the same - pre-filters, RO membrane and post-filters. Let's start with the queen - the membrane.

RO membrane can filter out pretty much everything, except dissolved gases. You can look it up online, but in short, imagine many many many layers of media, each layer is a mini filter getting rid of impurities that are washed away into the drain, the membrane is usually good for 2 years. I'd like to stress that point IMPURITIES ARE WASHED AWAY. They do not sit inside the membrane. Keep that in mind, it's EXTREMELY important for further discussion. What comes out of membrane is the final pure drinking water.

But, there is one thing membrane is scared of, and it brings us to our next point - pre-filters. Chlorine in our water can destroy our queen membrane in no time. So we need to get rid of chlorine first. This is why pre-filters normally are sediment + carbon block x 2. Sediment doesn't let large size particles to carbon blocks , 2 carbon blocks absorb all the chlorine, the rest goes to the membrane. Very simple and straight forward.

But then why post-filters? Membrane produces clean drinking water, what else is there to post-filter? Remember the tank that most RO systems has. Water sitting in the tank starts smelling and tasting bad. Tank also has bacteria inside, the bladder leaches plastic into the water, so post-filters are designed to get rid of all that. If there is no tank, there is no need for any post filters.

For 2500RM you can get quality tankless RO system with descent flow and it will outperform any non-RO system ever created. At 2000RM or so you can have quality RO with tank (though maintenance cost will be higher than tankless because of post-filters).
So now you know how a system and a price range that can get you purest drinking water, compare it to your current system or the one you want to buy. Start from the left to right. For example:

Does your system have sediment filter that you can easily wash or change? Yes - great. No - all the large, elephant size particles flow to your next filter and may either damage or block it. Does your system have a carbon block to get rid of smell, taste and chlorine? Yes - great. No - hmmm, what do you have, just a glorified sediment filter?. The simplest system would be just these 2. And it must be relatively cheap even if it contains premium cartridges and other premium components made in USA/UK, we're looking at roughly 700RM for the system and 150RM worth of cartridges per year. Also, carbon block can be modified to filter out lead and some other heavy metals, but even after heavy modifications, carbon remains carbon. It cannot suddenly become RO membrane.

But then it gets a bit tricky. Bacteria, viruses, other heavy metals, plastics etc. How are these filtered? It MUST be a filter able to remove sub-micron particles. In other words, light version of RO membrane. There are few ways to achieve that, but filter manufacturers must clearly say how they do that. You don't have to understand it, but explanation must be available. Electroadhision, ion exchange, ultra-filtration through hollow fiber etc. Not just "hey bro, magic happens inside and you get pure water". Cost of these advanced systems should be 1000-1400RM MAX!, cartridges for a year are about 350-400RM. If you're paying over it, you're getting robbed. Remember that 2000RM is a price for RO with tank, so why would you pay more for less clean water? Even if you're still, for some reason believe that "must have minerals bro", just buy a mineralization cartridge for RO. Do not ever, ever pay over 1500RM (ok, let's put it 350USD, who knows where RM will be tomorrow) for non-RO point of use system. And this is only for the one that is actually able to remove all bacteria etc, not just simple carbon+sediment.
One last, but very important thing to keep in mind is lifetime of the filter. Remember that with RO membranes, impurities get washed away constantly and yet it can last only 2 years? Any other filter that doesn't get washed can only last 1 year, no more. And sediment needs to be changed or washed few times per year. There is no way any filter directly exposed to the incoming water can last even 1 year without any kind of wash. So you know that all these "black box" filter systems get dirty quickly and stop performing.

This is it for today, hope at least few people will read it.

Ok, this time I will post without any links or any sales.

My background is water treatment. I worked in Canada, USA and South America on various water treatment projects mainly RO projects though. I was also appointed by one of my employers to go to establish manufacturing in China, which didn't go well, but whatever.

So what a good home system must have? Right now we're talking about 3 stage system only. Why 3 stage? Surprisingly, it's the most economical and easy solution. You can do 2 stage as well, but it's not worth it, I'll explain why.

Stage 1 MUST be a sediment filter. It's cheap and it takes the first hit and you should replace it often (3-6 month). There are 3 types, string, pleated and melt blown. Just use the melt blown one. Cost for a good, USA made one is roughly 20 RM, change it 3 times a year. So 60RM yearly for this one. Choose 1 or 5 micron (I prefer 5 for better flow)

If your system doesn't have a sediment filter as a first stage, then all the large kaka goes to your first filter and reduces it's lifetime and performance dramatically.

Stage 2 is your main filter. Normally it's sub-micron, hollow membrane or other technology (in USA they don't even make UF, they have other sub-micron filters). This is the heart of your system and it's also must have. Make sure it removes: bacteria, viruses, cyst, lead, arsenic, ferrous iron, mercury, pharmaceuticals and plastics. Pretty much everything except: fluoride, chlorine, bromine, iodine and VOC. Change once per year. Roughly 160-200RM for a very good one (US made)!

Stage 3 is your carbon block. This will remove chlorine, bromine, iodine and VOC. Change once a year, roughly 80-100RM for a very good one (US made, coconut activated carbon).

So you're looking at 300-350RM per year worth of outstanding quality filters.
So see the logic here: first, remove large particles. Then, remove all nasty stuff such as bacteria and heavy metals, then get rid of bad taste and odor.

What's left? Normally it will be minerals and fluoride. But, sometimes you still have a bad taste, how come? Answer is chloramine. Nasty nasty stuff. Widely used in USA now instead of or together with chlorine, but can also form if ammonia is present in your water. You need a special chloramine carbon block and these are not cheap. EVEN THE BEST SYSTEM without chloramine filter will produce nasty water. Chloramine is not removed by any UF or sub-mic or anything else. If you have chloramine, your cost of carbon block (stage 3) becomes close to 160-200RM.

So say you don't have chloramine and let's assume you don't have any radiation (unless it's well water, hopefully not). Then what's left is fluoride. They do add crappy fluoride to our water in USA and in Malaysia (Western Europe banned it for example). You need a special filter if you want to get rid of it, so an extra stage. I personally don't care, but choice is yours.

Now to plastic leaching. If components of your system (not filters, but everything else) are made God knows where, then there are high chances that they leach chemicals into your water. Anything before stage 2, we don't care, since our stage 2 will take care of it, but anything after it goes straight to your cup. So know where each and every COMPONENT that touches the water is made. (example, does your faucet contain lead? if yes, then what's the point if filtering out all the lead before?)

Last but not least, I HIGHLY recommend a system with standard housings and filters. We call them 2.5 by 10, 2.5 by 20 and so on. Even if the manufacturer of your system jacks up the price for replacement cartridges or goes out of business, you can always buy cartridges from reputable US companies at reasonable prices.

This is it for now. If you have any questions, please let me know