Rad is great based on two of its AoEs, but two ST attacks have longish animation times and those keep it in the second tier for me in terms of fun. Sets I got to 50 although not on tanks which are fun for me would be Claws, StJ, and Kat. I'm at level 30 with EM. While it might lack a bit in the fun factor, it is good enough I think I can default to using it when I pick a new build. Savage and Psionic could be top tier for fun if they got reworked(Hemo, Insight). Also working on a Dark/staff which has been fun. I probably have to turn in my Tanker card, but SS is one of the most boring sets I have played. Despite being what it looks like on paper, I couldn't make a SS/elec fun. Slow sets in general don't work for me either.
I've only hit 50 with WP and Bio for sets tanks get. I've only IOed out the WP and currently working on the Bio. Current leaning is Bio is better for the fun factor. I have some Rads, Darks, and Shields at lower levels, but I don't find click mezzes fun. I'll know more when the Rads and Darks get IOed out. Non tanks, EA all day for me. /regen scrapper was great until late game although I never IOed it out either.
If I had to pick a build for min/max + fun, I think I would go Dark, Bio, or Rad with EM. I definitely think about a Dark/em for the stacked stuns.