Mostly series of games, heh.
5. Metroid Prime (Series)
Had a lot of fun with these games, played them on the Metroid Prime Trilogy for the Nintendo Wii, which sure was a fun Summer.
4. Turrican (All of them)
Now this series right here got me into music composing, and game design.
3. Streets of Rage (All of them)
Another one I had fun with, and another one that got my into music composing.
2. Double Dragon II NES
By far my favorite NES game of all time. Real fun, and has one of the best endings I've ever seen in a game.
1. Fallout 3
Words can't describe, to me, this game has every thing, and I find a little more every time I play. I could talk all day about it, the story is great, (While it mostly has elements from the first 2 Fallout games, it blends all together, successfully reintroducing the series) the entire soundtrack is good, the secrets and items are good, exploring is fun, and overall everything is really satisfying. Fallout 3 is probably the most memorable game I've ever played.