I'd have to say my favorites are the Vivi slim tanks, T3s, eVOD is pretty good but they don't thread well onto my devices for some reason, maybe it's just MY evod, and most recently the iClear 16. I find myself using the T3 the most, but it's so easy to clean and change flavors in a ViVi it's nearly a tie with the T3. They're all pretty cheap, and replacing the head/coils are cheap too (much cheaper than carto tanks I've found in the long run).
I do have a ProTank in the mail, but no experience with it yet.
Buy one of each, try them all, prosper?