I believe The Beatles: Rock Band came the closest to being perfect. Eveything about that game was just beautifully done and the only things missing was Pro Drums, an option for Keys, and a few more Beatle songs (Hey Jude, Strawberry Fields Forever, Yesterday etc. etc.)
Gotta be Portal 2. The puzzles are fun, the difficultly curve is reasonable, and the writing is fantastic. Truly the only complaint I have is that I wish there was more of it.
What I really like is that it is never unfair. Once you understand what has to be done, it is easy to accomplish. So it is a great kind of difficulty. And there is more: The free mod “Portal reloaded” is really great.