1) CHUG (of course)
2) Prime
-- The show is really great, and while the line has some...questionable molds, they're mostly pretty good, and some of them are spectacular!
3) Armada/Energon/Cybertron (Mostly minicons from Armada, the bulk of bulks

from Energon and Cybertron)
-- I have a big of nostalgia for Armada, but the toys don't hold much interest. I do however love minicons! The Energon line's combination gimmicks are and easy draw; the only thing better than a giant transforming robot is a giant transforming robot that combiiiines~. Cybertron just has some spectacular molds, like Downshift (Wheeljack homage wrapped up in a muscle car

), Metroplex, and of course Optimus, who I NEED to own at some point.
4) Beast Wars/Machines
-- So so many great molds I can't even begin. But it's the Beast era, and it's all great stuff.
5) Movie lines (I just need Commander Hatchet for Dotm CV Decepticons dammit D:)
-- Despite all the hate they get, there are always a few gems in each line. 07 Dropkick is just a gem, all giant Decepticon symbol and gerwalk truck mode!