For me, the SQuape R[eloaded] is the best thing going today. I've had a hard time with the RTA's I've tried - probably due to lack of skill, admitted, but I couldn't get Kayfun and ErlPrinz to stay leak free. Especially the Prinz had a temper. Would vape perfectly for a good amount of time, then all of a sudden, liquid all over.
Then I tried a SQuape R clone and got it working rather well. Bought an original and my, oh my, vaping bliss. Great flavor, easy to build, zero leaking issues, even when down to it's last drops. So I bought 3 more and got rid of my other tanks

It's a pretty big tank and also rather heavy, due to most of it being stainless steel. But that also makes it solid. The glass window is well protected and they haven't been damaged from the few "drop tests" (which weren't really tests) that I've put them through.
It holds about 5 ml of liquid (I think data sheet says 5.2) and I love how you can adjust airflow easily without disassembling anything, just turn the air flow ring and it clicks into place on the air hole of your liking. Being able to check your build and wick without draining it, is also a big plus for me, as I vape a few liquids that are rather tough on the wick and coil.
So yeah, for me, it's the SQuape R, hands down
Edit: Oh yeah, downsides - price. It is a rather pricey tank, but to me, it's well worth the money, when compared to ease of use and peace of mind it gives me.