I just wanted to chime in here and say that after buying and rebuilding on MANY RDAs and RDTAs at this point, (including the mutation v2, dark horse, uwell crown, smoktech tpv4 and tpv4 mini, kayfun v3 etc, avocado, various velocity and Kennedy clones, twisted messes and TMv2, Kanger sub mini, herakles, and several others), I've found by far the easiest build with no leaking and most forgiving wicking setup to be the authentic Geek Vape Griffin. The 22mm (original) is my fave, though the 25 with top airflow is nice too, if cumbersome. The build deck is very large and easy to work with, and you can build single or dual coils. The ceramic plug thingy to build single coil that's usually labeled for the 25 (and comes with it) fits BOTH RDTAs, so get yourself a 5-pack of those replacements for about $5.
I've had terrible leaking issues with almost every RDTA I've bought, despite endless hours watching YouTube videos. No matter how identical my wicks look to the video, I seem to be plagued with leaks. The Griffin is the ONLY RDTA I've had ZERO leaking with, and I love it. I typically build fused alien coils or fused claptons, and the decks are so easy to work with I never have space issues. They provide generous extra parts as well, and the taste is awesome. I love having juice flow control, and both tanks have this (22 and 25).
My runner-up is the SmokTech TPV4-mini with its RDA deck (included in the later packagings of this RDTA and available separately).
At about $25 each, the original Griffin is a great deal, and I highly recommend it. Fasttech is a good source for all.