Good question indeed! I collect, well, too much. I've had to draw a line in the sand for the time being due to space and money, otherwise I would be sorely tempted to move on to collect He Man figures - both MOC and loose, as they were big favourites as a child. Eddie - I notice your collecting tastes are as completist as mine, if not mroe so, so I would heartily encourae you NOT to bracnh out into other toy ranges - it is very addictive!
I keep meaning to post some updated pics of my other collections, but I never seem to get a chance to take them. MOC I have BSG, Dukes of Hazzard, Clash of the Titans, Battle of the Planets, CHIPS, Black Hole, Flash Gordon, Buck Rogers, Terrahawks, a few Star Wars, and, of course, Action Force up to series 3. Most ranges are complete.
Loose, I tend to limit my collecting to just AF and I am planning on resurrecting the collecting of loose Star Wars figs eventually, to try to complete them loose.
I also have a vast collection of 1/18th scale diecast cars. Had to stop with them as space ran out. So, for me, space is a big factor now. I've been subtley pruning my collections over the last year or so, and this continues at the moment, with me seriously considering selling on my Black Hole and Buck Rogers collections in the New Year. Not that I dont love them, it's just that space is a real factor now, and the shelves brimeth over, and those collections I never really had as a kid, so the nostalgia side of it isn't there as much. We shall have to see.
My favourite items I would say are either my 1/18th scale General Lees signed by Cooter, Bo & Luke, or my Trans am signed by Burt Reynolds. AF wise I would obviously now have to say

is my favourite for the adventures he's been on with me lately, but nostalgia wise my

holds a special place in my heart as my first ever AF figure. With his glued leg and all he still has pride of place in my cabinets!