I like the Elder Scrolls series, Morrowind is great. My computer isnt good enough to run Oblivion but I may get it for the 360 some time. Am getting Assassins Creed and COD4 for xmas so hopefully they'll be good :)
Not sure what I'd say the best ever is, but Goldeneye 64 is a classic. I really liked the Commandos series as well, I never completed more than 2 levels on Commandos 3, it was stupidly hard.