In chronological order:
D&D - 5th edition, as it stands. My first game, and the most accessible towards intro play.
Traveller - Classic or Mongoose. Love the game-within-game systems, and the sheer diversity of adventures to be had.
RuneQuest - 6th Edition is fine by me. Flawless game world simulation, with a spiritual core.
Call of Cthulhu - l’m a bit un-plussed by 7th edition changes, but otherwise as is. Love the diametrically opposed premise in game design to ‘heroic’ D&D. Face it, things aren’t going to end well….
Paranoia - In my view, the prototype ‘indie’ game. Provocative, sardonic, satirical, and mandatory fun!
Mage: The Ascension - Possibly a ‘guilty pleasure’ game as the rules were sometimes sloppy and the premise may be a little pretentious. Loved to play it throughout the 90s though, and made The Matrix seem like a watered-down substitute when it came out - like Vampire: The Masquerade compared to Underworld.
Feng Shui - Another Matrixy-game, released before the movie came out. Instantly playable and versatile though. Completely filled the ‘action movie’ niche for me.
Doctor Who: Adventures in Time and Space - absolutely flawless design, very easy on the eye, and perfect for running TV show inspired games.
The One Ring - another ‘perfect’ game design, totally suited to emulating the source material. It’s the modern Pendragon for me, in some ways.
Fiasco - Classic indie design…endlessly adaptable…but without the subtext that takes itself too seriously.
my alternative game of choice.
Other notable mentions (but in some cases with, to be honest, very limited actual play): Baron Munchhausen, HoL: Human Occupied Landfill, Amber Diceless, Ars Magica, Pendragon, Ghostbusters, Toon, Vampire: The Masquerade.