I can't number them, but here's ten that absolutely stand out for me:
Deus Ex
Grim Fandango
The Longest Journey
Super Mario Galaxy
World of Warcraft (Classic and Burning Crusade; Wrath of the Lich King sucks)
Ikaruga
Star Wars: Knights of the Old Republic
Mass Effect 1 & 2
Starcraft II
Hitman series
I also absolutely love the Assassin's Creed series but it doesn't fit into the best 10 of all time for me. Deus Ex, Grim Fandango, and The Longest Journey are still my three favorite games of all time. I am still waiting for the Dreamfall chapters to come out, but they're on hold indefinitely until Funcom finishes their new MMO
Super Mario Galaxy is one of the best games I've played in recent memory. I had an absolute blast with it; the exception are some of the purple coin challenges but I did end up finishing the game. I have intentions to get the second one but I don't have time to play games right now.
WoW -- I have invested over four years into that game. It's a love/hate relationship. I hate the game now, but I cannot deny that I had a lot of fun with the game before Wrath came out. After that, the game just went downhill and now I have zero interest in it. That said, the good old days were great.
Ikaruga is one of the best games I've played. It's simple, but there's a hidden depth to it if you can get past the steep difficulty curve.
KOTOR has an awesome storyline, even if the gameplay was a bit flawed, especially near the end. It is one of the best story in a video game ever told.
Mass Effect 1 & 2 are absolutely awesome. The personal element and the rich storyline and characters make up for any deficiencies in the gameplay.
Starcraft II - it's the only thing I am playing right now. The online play is so much fun. The Starcraft series is also really the only RTS games I play.
Hitman - flawed games in general, but I love the concept of the game and am always looking forward to the next one.