Although I will probably get flamed over this. I will always be able to go back to halo 3 and play. I love GTA 4 and COD 4 but halo 3 will always be most enjoyable.
Halo or cod4 . For the FPS category. Lost oddysey or Fable. For the RPG. Racing games are all simular enough but probably the newest ones are the best. For anything else I choose devil may cry 4 or something else >.<
There are several games that come to mind when I think of the word "Enjoyable".
I loved The Elder Scrolls IV: Oblivion, and I always will. I loved Gears of War, and I LOVE Call of Duty 4. I personally though, think that The Elder Scrolls IV: Oblivion is the most enjoyable.
Oblivion just has so much free roam, dungeon crawling, creature killing, and sidequests to keep you held over. The DLC is awesome, such as the Shivering Isles, and the Knights of The Nine. I also think that the game has a lot of creativity in it. It does have some glitches and such, but it's my nominee for 'Most Enjoyable'.