I often times wish I could collect a few more lines than I am able to, but for now its just a little bit:
Vintage Star Wars 3 3/4": Loose, complete figures
Modern Star Wars 3 3/4": Figures, Beasts, Vehicles - Only the ones I like, not a completist, I buy them carded, but I open them as soon as I get them
Marvel Legends: I just got into this line, only collecting the "X-Men" ones.
State Quarters: My grandma started me on this collection, and gives me all the ones that come out from each year at Christmas
I've been "tempted" by a few other lines along the way, but haven't bought into them yet (we couldn't afford for me to collect something else I don't think, plus we don't have the room). Other lines I almost got into include LOTR, the rest of the Marvel Legends, Gi Joe, Simpsons, and the new Masters of the Universe. Just so many great toys nowadays