My first sets were Futuron, but those figs and pieces were pretty beat up by the time I really got into LEGO. That was when Blacktron II, M-Tron and Space Police were out. I had small and mid-size sets from all of them.
My golden age, though, was made of Ice Planet, Spyrius and Aquazone (who I integrated with space, sort of like the dudes who explored a new planet's oceans). I still liked Exploriens and UFO, but then basically got out of LEGO for years after that.
I have really grown to just appreciate all space themes. Blacktron and Classic Space are great, but I think Ice Planet, UFO, Blacktron, Roboforce and Alien Conquest are all just about equally awesome in different ways.
Truth be told, the only ones I'm iffy on are the two Mars themes.