I have tried so many of the tanks listed in this thread. The Erlkonigin was probably as close as I got to replacing the kayfun lite v2. I just couldn't get the airflow to cooperate. I'd get it where I liked it, heavily restricted, and I'd end up burning the wick. Open it up, and it was too airy and prone to leak.
The Aqua 2 wasn't bad, but it was a juice hog, being a double wick design. The GP Heron is a good atty, but prone to seepage if left sitting too long. That can be countered by filling it with the blue foam, but that's a bit of a hassle.
So I just stick with the kayfun lite plus v2 and aftermarket bell style tanks. I tend to buy the bell tanks 5-10 at a time. They last until they've hit the concrete a couple times. I'm a firm believer in less parts=less trouble.
Jumped on the Kayfun 4 bandwagon with all the other lemmings, and just couldn't feel the love. I felt it was over engineered and too complicated. From what I've read about the Kayfun 5, it should more properly be called the 4.5, as it's a stripped down version of the 5 that keeps the good things and ditches the bad.
Kind of like the 3.1 vs the Lite, now that I think about it.
@The Ocelot had/has a favorite tank as well, but I can't think of the name offhand. If I remember correctly, it was a similar design to the Kayfuns, but had a 1.5ml tank. Not a popular atty at all. Before that she was using Vivi Nova's. Heh. Been a while since I thought of those. I remember when they were THE tank to use on a ProVari.