It's hard to say which one I've enjoyed most. If you want total play time, then 90 hours on Oblivion is quite a good bet, followed by Devil May Cry 4, which despite its shortness does have a surprising amount of replay value, along with the escapism factor in that game being "God I'm so badass while I play this".
If you want real escapism, then Gears of War 2, and to a lesser extent its prequel are surprisingly good. Whilst I didn't buy into the whole tradgedy of the thing completely, I was saddened by some of the events, a first since Final Fantasy VII.
Probably the most universal, non marmite game for Xbox 360 owners is Halo 3. It does nothing its prequels haven't done before, but its a fun, colourful varied and occasionally challening blast, perhaps giving 12 hours of gameplay on the first playthrough on legendary.
Bioshock is surprisingly absorbent, and GTA IV is also a good one, despite the flak it gets. However, if I really had to pick one, I'd go fo Gears 2, if not for the story, then for Horde mode.
Whilst I would say The Orange Box is good value, I wouldn't say its the best game on 360. It still feels too much like old material. Whilst I do look forward to Episode 3, I am well aware that it will feel dated compared to some modern games. Hopefully, Valve will have to work more magic on it than they have done with Episodes 1 & 2, which I feel are very much in their older brothers shadow. And whilst Portal is indeed a good game, it is too short to takje away from suck titles as Bioshock and Mass Effect. A longer game with varied environments would tip the balance in its favour, but right now, I consider it comparable to Peggle and Plants Vs Zombies.