Hmm. Let's see...
-Masters of the Universe. Love, love, love this line. My first action figure was Skeletor, and it was all downhill from there. I have an almost complete museum-quality vintage collection that's easily my favorite part of my collection. I haven't really gotten into the vehicles in this line because I want to buy them with the boxes. That box art was the best in the business.
-GI Joe. I've collected ARAH, the JvC/SpyTroops/VVV stuff, and just about every figure since the 25th line started. And the vehicles too...
-Star Wars, vintage and modern. Wouldn't be here without them, and I swear collecting modern Star Wars has become so ingrained in my routine I don't know what I'd do without it. Is that weird? I can never tell.
-Transformers. I can have these now that I promise not to break them. I really like my G1 collection, mostly reissues. And the modern stuff too, though with these, I really need to go through and get rid of some.
Then there's other stuff I've collected, like Lord of the Rings and Marvel Legends, but I don't put these in the same class as the above lines.