Gentle Giant also would have to be the top of the heap. The statues are nice, though I prefer the Sideshow versions, there's at least more variety between the Rancor, Dewback and other items. The mini-busts were already mentioned and I'm sort of selectively keeping up now too. Then there's the bustups, between Simpsons and Star Wars, they're pretty sweet. If it's only one subset of the group, then it would be the mini-busts.
Kubricks are right up there too, could certainly afford all of the carded versions if that's all I collected.
Vintage Hasbro would be pretty sweet as well, tons of choice and variety and you'd have a hard time ever being complete.
Modern Hasbro? Not even on the radar.