(18) Favorite game SYSTEM
It's a toss up:
2d20 is a pile of moving parts, but it runs nicely and builds up speed through a session. 2d20 adapts well to a variety of settings from Starfleet officers doing space stuff in Star Trek Adventures to the savage violence of Robert E Howard's Conan. There are some tricky bits - spaceship combat is a hurdle for any system - but I enjoy it.
On the other hand:
GUMSHOE taught me how to manage a procedural adventure. Information might give the PCs an advantage, add flavor, or be nice to know. And there are things that don't make sense to gate behind a dice roll - what the players need to know to get through the adventure.