I find I really enjoy all the sizes so far lol, I love 18350 for the concealability and ease of carry, 18650 for the form and battery life, but I have gotta say, I've been using this 26650 for about two days, and I'm really digging the size. The 30mm rba looks like it'll be a breeze to build on once I need to change these coils, and it's easy as all get out to wick. This rba holds a LOT of juice. It's kind of a pain to carry if you don't have a big pocket like in a hoodie or jacket, but it just feels at home in my hand. The button is smooth, the locking ring tends to lock up on me, but I might just be overtightening. The button seems to have the tendency to loosen the bottom contact pin, but it doesn't do it often enough to be a problem.
Anyway, I'd like to get my hands on a 26500, I bet I'd like that even more...