DIR-850L not suitable for double storey house. i tested already, i place it on 1st floor, ground floor signal very weak, some place cant received the signal at all.

Asus rt-ac68u, if you have the budget.

Latest may not be the greatest. I am using ac88u, rather unstable, even my old rt-n66u is better.

how big is your house? my house is 2500sqft double storey, i am using Archer C2600 router place at middle of 1st floor, 70% area can get full 100Mbps with 5Ghz, the rest at least 50Mbps with 2.4Ghz. Tested the  Asus RT-AC68U  Too, almost the same result. Archer C5 wifi can cover the whole house, 60% area an get 100Mbps, the rest between 20-70Mbps.

The speed may differ by router location and house layout .

Wouldn't say it's the best, but better (stability, coverage) than a few newer and more expensive Asus release. You can read about it on snbforums or hardwarezone.
Read good reviews about higher end tp-link and synology, you may want to consider. With the size of your house, you may want to consider mesh network?

No.wifi mesh is like intellligent range extender that auto connect with each other without u configured it.

No, AC is worst indoors due to higher frequency used. Signal is attenuated/reduced more by walls.

if speed above 50Mbps and wifi still using non AC routers will give different result whether speed test or usage with wifi AC routers...that's why we ask..who knows maybe he still use older router provide by tm. well maybe urs consist technical probs outside the router...normally with tm side...

I mean the old stock router(silver one) provide by tm to users cannot reach 47-49Mbps during my 50Mbps. the highest 32-35Mbps.....so since a lot of us getting 50Mbps connection and still using old router, maybe u guys can go end search for more reliable and cheap router in the market that support better wifi connection. for new customers they already gets basic ac routers DLINK or TPLINK.

lazada sell cheaper, bought one before RM799, with citibank get another -RM25 on weekend
but i not sure the link you mentioned is it newer hardware model, the one i bought from lazada still have the back ASUS LED which is first gen AC68u, latest one dont have already but the Asus form at lowyat the TS mentioned Malaysia dont have latest model selling, only oversea if not mistaken

now even cheaper i bought from this seller previously http://www.lazada.com.my/asus-dual-band-gi...8u-7632013.html

Isn't that official Asus store? Still, you got a point, can they be trusted?

I recommend Mikrotik.

If you want best one, I recommend you use Fortigate, then add another Ruckus AP, done.

Sure, Mikrotik router + enough APs to cover the house. I think i already suggested a few months ago. As opposed to 1 super-duper powerful one.
Not expensive, my friend is using the RM130 hAP Lite model for his 30mbps. Yes, if you have a nice house, you might prefer Unifi because the thing that I dislike most about Mikrotik is their AP design. Unifi is much nicer. You can mix and match, no worries about compatibility.

Agree. Once you master the basic of configuration of routerOS, you should able to handle consumer router in market 
Also my mikrotik serve me since 2014 and still kicking from 10>20>50. Awaiting for 100 

ur house area consist 5000sqf, u need atleast AC1900/AC2600.....and your budget will be a bit high....

Its 951G-2HnD. I cant remember the price. less than rm 300 perhaps

Agree. For my house, 1st floor use AC while 2nd floor just normal N. time2 tido x elok internet laju

Says 802.11 b/g/n...