1. Mechwarrior 4: Mercenaries (PC)
Hands down my favorite game in my favorite game series of all time. Blowing up mechs in order to earn money to buy better mechs, I'm in. With the recent free release and addition mods added with the free release, this game continues improving.
2. Marvel: Ultimate Alliance (Xbox 360/PS2)
One of my favorite games, that's carried into 2 different consoles (also enjoyed it on the PS2). It's one of the few games, I can go back and play for a couple hours and still want to play a little more.
3. Tony Hawk's Underground (Xbox/PS2)
I enjoyed the previous Tony Hawk games, but this one had a story that kept me playing long after I'd gotten bored with the others. In fact, I still throw it in my 360 from time to time.
4. Assassin's Creed 2 (Xbox 360)
I enjoyed the first game, but the second one completely blew it away. This one felt less like a chore and more like a game when I was playing. I kept playing much longer than I logged in the first game.
5. Rock Band 2 (Xbox 360)
Everyone at some point has wanted to be a rock star, but few have the time and commitment it takes to be a star. Sure Guitar Hero came first, but Rock Band feels more like you're playing the song, as opposed to playing as many notes as can be fit in.
6. Uncharted Waters (SNES)
My friend and I would play this for 16 hours at time, sailing the seas, trading for gold, and looting other ships. Although many don't remember (or don't want to), I found it incredibly enjoyable.
7. Combat (Atari)
Small, simple, and to the point. You have your blocky tank, plane, or jet... and you shoot your friend's blocky tank, plane, or jet. Doesn't get more simple than that, and it's a lot of fun too.
8. Burnout: Revenge (PS2/Xbox 360)
Racing is fun, taking people out is fun. combine the two and you have a winning formula. I chose Revenge over Paradise, because a game like this is best enjoyed with friends in person.
9. Street Fighter II (SNES/Genesis/Arcade)
The first fighting game I really loved. I remember playing this for hours with my friends and in arcades. Sure I sucked, but I never enjoyed losing more than I did playing SFII.
10. Area 51 (Arcade)
On rails light gun shooter, probably my favorite in the genre and I game I spent hundreds of dollars playing in the arcade. I enjoyed getting the shotgun... just saying.