Oooh, tough to limit it to 10, I have a lot of old games that I constantly play to this day. At this moment, these are the 10 I can probably assure you I"ll still be playing in 10 years, excluding MWLL from the list for obvious reasons.
Freelancer -- Oh god, this game is amazing. Years ahead of its time, and still has an active modding community.
The Elder Scrolls (entire series) -- Great games, with a world that just draws me in and won't let go. Mods make it better.
Left 4 Dead (1 & 2) -- Anytime you have 1-3 friends and want a great couple hours of gaming, L4D co-op is there. If you have more friends that don't mind the hilarious griefing that is VS, there's that too. Random matchmaking is shit though.
Legend of Zelda: Ocarina of Time -- absolutely groundbreaking game, I go back and play through it again at least once every couple years. Nothing will quite capture the feeling of completing it the first time though. Master Quest on the gamecube was a fun re-visiting of it with new puzzles that made it quite refreshing actually.
Eternal Darkness: Sanity's Requiem -- Not something I can play every day for years, but it's a great experience that deserves replaying every now and then. Sadly the sanity effects don't fool me anymore, but it was INCREDIBLE the first time through.
Amnesia: The Dark Descent / Penumbra Series -- If you haven't played Amnesia, go play it. Now. Much the same as above, best on the first playthrough for all of these games, but still deserves a special place for me, as I've never played a game that has made me scream like little girl before.
Halo (series, at least, up until Reach, I doubt the non-bungie titles in development will be anything but shit) -- Gets a lot of grief because of the shitty playerbase, but Bungie has a way with storytelling that is only matched by other great game developers like Valve. Plus, the music is absolutely brilliant and atmospheric. I really can't wait to see what Bungie does now that they've passed Halo off to someone else.
Marathon 2 -- Basically all the great things that make Halo awesome in story and atmosphere, wrapped up in a Doom-era shell... with crazy AIs. What more could you ask for?
Final Fantasy IX -- Best of the series, if you ask me.
Chrono Trigger -- Will never be surpassed in its awesomeness. The best damn JRPG ever made, best story, best characters, best music, best gameplay.
EDIT - Read the last page or so of the thread, and for the record: SNES > Genesis. N64 > PS1. Xbox > PS2 > Gamecube. And the current consoles are all crap, though I do own a bricked 360

Oh, and PC > all.
EDIT Again:
Sega was great if you were a Furry 
Don't forget Star Fox, especially Krystal from Star Fox Adventures... which is entirely Nintendo.