Just going by the new variants, my faves are definitely XPW Captain Cosmic and Visionary Unleashed. They turn two heroes I would rather not play into heroes I absolutely want to play. I also adore Extremist Sky-Scraper.
I wouldn't say there are any overall I hate, though I tend to avoid playing DW Setback, PW Tempest and F6 Tempest. Also, I'm not a fan of how powerful Hunted Naturalist is. I've said before, you start playing him, and then regular Naturalist feels slow and clunky by comparions. :(
Xtreme Argent is also not a favorite of mine, save that I find I prefer him if I have to play Argent Adept in a team match. Being able to deal damage is far more useful than constantly having to set up, and you'll always have sacrificial ongoings! :D
I, too, have considered writing strategy guides for various incoming variants, but I haven't figured them all out. Specifically, Xtreme Haka -- Powerhound showed off that combo with Guise and the Scholar, that's the first way I've seen his power be particularly useful -- and Freedom Five Wraith. I mean, all the other Freedom Fives, with the possible exception of Tachyon, get excellent powers that are obvious upgrades or equally as awesome as their other power. Clandestine Funding I just cannot figure out how to turn into an asset. What's the upside to it? Guise can combo it to draw his whole deck, but otherwise, eh.
Speaking of Guise, Completionist Guise is stupidly powerful, and every time I figure out a good combo with him -- synergy with Captain Cosmic, the Fanatic Blender -- I feel really smart. :D
As for variants I wish existed, A) I wish everyone had at least three (including base), for better utility with Completionist Guise, and B) all the alternate versions of characters we've heard about in the Letters Page and seen in the ARG. Especially things like WWII Absolute Zero and Lazer Ryderz Captain Cosmic! (That's what we have customs designers for...)