The amount of Portal votes doesn't surprise me, but I honestly think there was nothing "special" about Portal. Sure the gameplay was cool, but the graphics are shit (the Source engine in general has pretty awful graphics at this point, L4D2 could even look better on other engines), and honestly I never laughed during that game. The "humor" was (IMO) pathetic. Otherwise, the game's 2 hours of play was fun, but that's it. 2 hours. I can play Zuma, Bejeweled or Tetris for longer than that and have more fun.
Anyway, I don't really have a 10/10 game, but games that come pretty close for just good memories and no real bad ones are:
Jedi Knight series
Prince of Persia: Sands of Time
Jak & Daxter + Ratchet & Clank (I put 'em together because, for me, they are extremely similar).
Quake 3/QL
America's Army 2.0-2.8
Call of Duty: Modern Warfare's MP/Call of Duty 1's SP
Halo 3 MP (ODST's SP too if that counts)
Mostly because I just loved playing these games with friends, or loved the atmosphere and gameplay mechanics.
GTA has a place in my heart, but only because you can wreak havoc in any way you choose. The games' stories, graphics etc. have always been sub-par (GTA IV was pretty good though, the physics engine in particular added to the fun-value), so they don't rank up there.
RDR I would've placed it on this list if the MP wasn't so horrible. SO MANY CHEATERS ruins games.
CSS is high up, but as I said graphics aren't the greatest (NOTE: I don't put graphics above everything else, but for games that try to be realistic of sorts, it hurts the experience when they aren't the best they can be, for me), and it's not the best game on its own; it's the mods that make it so great.
And those are my favourite games lol.