Now of course what ring you can buy depends on your budget:
1) You rich (Tiffany, Cartier, Bvlgari) - you get a ring here, everyone will be in awe - here are top quality rings with of course top level price tag. Don't buy if this is not in your budget (by this I mean, don't blow your annual savings or go into debt just to get these) they are vanity brands. Budget (30k - 100k or inifinity also got)
2) You premium - middle level brands with middle level prices (8k+), rings here there's exp but just not the top quality. Cut is pretty good, and decent shine and brilliance. Your SO will be happy at the quality even though there's no brand here.
3) Others - Let me share with you my experience after buying a diamond ring. Firstly if you are looking at around 0.3c (this is a good size, but relatively speaking its a small diamond) - so inclusions, color what not (all these are pointless because you can barely notice them) the only thing to compare is the "cut" because this dictates how "shimmery" the diamond is, and the brilliance of it. Just do an "eye" test, select from a few of that carat range and see which one is the best under the light. One thing that I will advise if you buying from Zcova (or any online), you NEED to see what you are buying first - you need to see the cut and the shape of the diamond.
Let me tell you for the inclusion and color, anything under the light you won't be able to tell a difference. If there's a cheaper option but with lesser quality (don't consider those bottom of the barrel grade la) like VS1 against VVS1 (go for VS1) or color E against H (go for H).
Lastly this is minor, but the setting of the diamond is also important. How the claw, or the clasp of the ring on the diamond will determine the entire design. So do check that out, even if you get the best value diamond for 0.3ct if they set it lousily on a ring, your entire ring will look like shit as well.