I've tinkered around with a few RTAs in the past week or so and my personal findings are:
Orchid V4 - Serious airflow. Close to RDA experience, got a 0.41 ohm dual coil build in here with a surprisingly chill vape. Can be leaky in the first few attempts building it.
Aqua V2 - It has a restrictive lung draw (my opinion, YMMV), I'm enjoying this fairly more than my Expromizer (ditched the AFC ring and drilled out all four air holes to 3mm). Absolutely love it in dripper mode though. Threw a 0.36 ohm build in there with Nichrome 80 26 gauge and its a chuckin'. Had a pinch of difficulty coiling it but wicking definitely wasn't an issue.
Lemo - one of the best single coil RTAs I've played with to date. Uber easy to build and fair amount of airflow. The overall size of this thing might put off some but I love its minimalist aesthetics. Just a shame that you can't enlarge the holes BUT you could drill identical holes just next to it if you really want to open it up.
Expromizer - as explained earlier, I drilled it out to make it less of a pain to do lung inhales but its still relatively restrictive. On the plus side, the flavor on this thing is immense even at 0.5 ohms and I love the fact you could fix your coils without draining the juice prior. A 3mm coil with a rump full of cotton in there will be comfortable, no dry hits, no leaks.