Favorite Game System
If you go by the game system that dominated at least two decades of my career and takes up something like ten feet of gaming shelves (not counting notebooks full of notes and two plastic office boxes of other stuff), I’d say Champions/HERO System. Thing is, I’m beyond burned out on it now. Sixth edition was a bridge too far for me.
If you go by the game system I play the most these days, it’s D&D/D20/Pathfinder. But my preferred versions of those games are either the older editions themselves (hello, 2e AD&D) or retroclones. The new edition of D&D looks pretty sweet and I want to take it out for a drive soon, so maybe that'll be the one.
If you go by my go-to games for ease of use/hackability, it could be Feng Shui, Cinematic Unisystem, or Ubiquity.
If you're talking about what I'm running right now, Dragon Age (and theoretically ICONS, though I don't know if that's going to keep going).
Make of that what you will.