I'm in the same boat as many of you. I don't necessarily consider it "collecting", but as far as which lines I was most into as a kid and gave up on...they are pretty much the same. I'll list them from what I have most of to least I guess (or close)...
Star Wars/GI Joe - I pretty much collected these at the same time, most of my SW figures came from the ESB and particularly the ROTJ lines...I was a little too young for the first Star Wars (New Hope, even though it wasn't called that) line, never got any of the POTF figures either. GI Joe it was mostly around the 83-86ish time I guess, although I do remember picking up a fig here or there later on (the 1989 Snake Eyes comes to mind). I had just a few vehicles for each line, and I think the figures actually integrated in some of my childhood play scenarios
He-Man/MOTU - I guess this might be up there with the other two as well, I had a pretty good variety of this line, and actually had (and still have) both Greyskull and Snake Mountain. I still have all of my figures in this line from my childhood, although not necessarily all the weapons. Playsets really made these fun to play with, along with the "Attak Tracks" or whatever it was called.
Super Powers/Secret Wars - A little bit of each of this line, although I probably had a little more of the Super Powers line. Figures I remember having off hand are Spidey, Wolverine, Doc Ock, Batman, Robin, Joker, Flash, Aquaman, Green Lantern. I still have most of these figures, in varying degrees of condition
Transformers/Go Bots - Again, little bit of both. I only had a few Transformers (they were a little more expensive to buy myself, and didn't get a whole lot as gifts). I remember getting my favorite, Sideswipe, as an Easter gift from my parents. Makes me tempted to pick up the Alternators one that is supposed to be coming out. I also had an Insecticon (Kickback?), Dinobot (the Triceratops), and a number of the smaller ones. This is a line that I kind of combined with friends', so we had more to play with then. I probably had more of the Go-Bots, simply because they were smaller...also had the Command Center thingy too for those.
MASK - This is a line (and short-lived cartoon) that I really liked, but it ended too quickly (at least it seemed to me). I had my favorite, the orange pick up with a guy named "Hondo" if I remember right? I had a few smaller vehicles as well.
Baseball/Football/Basketball cards - Ok, this was probably the worse....I had a TON of these, and still do have most of them. I sold a few shoeboxes full of "extras" at a garage sale earlier this year, just because we needed the room. This was probably a big of a collector thing for me, next to what Star Wars is now. I can't begin to imagine what I spent on these things. I got out of these in the early 90s (maybe 94ish), partially because I couldn't bring myself to spend upwards of $2 a pack for like 8 cards. In my heydey, it was less than a buck a pack for many, with like 12-15 cards per pack. A lot of good memories of those times though, all of my friends collected as well. It got to be too many brands too, when we were collecting it was Topps, Score, Donruss, Fleer, Upper Deck, NBA Hoops, Skybox, Pro Line? (football), and lines like that. Half of them probably don't even exist anymore. The last card I think I bought was an Upper Deck Chris Webber rookie card, if I remember right.
Comics - I had a fair amount of comics too, but haven't bought any (other than the ones that come with ML) for a looong time. Top three were easily X-Men, Spidey (various ones), and Batman. I also had quite a bit of Iron Man and some Wolverine comics.
Hot Wheels - I had a lot of these, as well as Die Cast Tractors (hey I grew up on a farm). Still have many of those, used to have a little vinyl carrying case for the Hot Wheels....I think those are fun for almost every generation.
As far as now, its pretty much been just the Star Wars stuff for me. I have been tempted by many lines, but haven't ended up collecting too much of it. Mainly, because we don't have the room, the extra collecting funds, or the patience. I have two LOTR figures (RA Legolas, Smeagol - bendy), two MOTU figures (He-Man and Sy-Klone, although I'd buy a Roboto if I ever saw one), and about three Joe figures (I bought two different Snake Eyes figures, and happened to get a Cobra Commander and Storm Shadow with them). I do collect some Marvel figures, just the X-Men ones for now (have Beast, Wolvie, Magneto, Colossus, and Gambit), and recently picked up the Spidey 2 Super Articulated Spidey. Star Wars takes precedence among the others, all the time.