Single Player: Borderlands 2. There is just so much variety in weapons, enemies, and enemy behavior. I've probably put over 300 hours hours into that game and I still play it from time to time.
Multiplayer: Killzone 3 for competitive and Resistance 2 for co-op.
Killzone 3 gave more tactical options in multiplayer than any other FPS I played.
As an Infiltrator, you could dress up as the enemy, sneak into their base, plant 3 mines, and wreck their shit before they even know what hit them.
As an Engineer, you could set up the best defense ever with mines, an LMG, AND a sentry turret. If you had the
high ground, you were nigh-unstoppable.
The other classes can do a lot in the way of support too.
In Resistance 2, the 3 classes are designed to force teamwork and the 8 player limit makes it so that the cooperation of the 2-3 idiots that show up in every game is not needed.