These are probably my biggest time wasters. Not necessarily my favorite, but the ones i spent the most time doing.
1) Oblivion (I was stupid with how much I played that game. Right after a bad time in my life, and I used this to help me get over it. I doubt I will ever spend so much time on something so pointless as a video game again.)
2) Red Alert (I didn't like it as much as C&C, but it was easier to play multiplayer. Plus it came with an editor. I created two commonly used maps that people still use today: Minefields, and Treasure Island)
3) Counter Strike (I loved playing this game, but was average at best. As it became more popular, I couldn't deal with the massive ammount of annonymous douchebags though.)
4) Doom 2 (first game I got for my parents new Pentium 60. Before this, I had an 8088 that wouldn't play any modern games. I even created at least a half dozen of my own levels.)
5) Command and Conquer (My favorite RTS of all time. So many ways to play, and so many effective strategies to win. You could win in 5 minutes if the opponent isn't paying attention, or you could slug it out and win the old fashioned way.)