If I had to choose one, I'd definitely go for the Humble Indie Bundle V. I mean, just look at it. Psychonauts is an all-time classic from Tim Schafer and company. Bastion has one of the best-told stories in any game ever. Limbo is wonderfully ambient and creepy. Superbrothers: Sword & Sworcery EP is downright hilarious and charming as all get out. Amnesia: The Dark Descent is perhaps the most frightening game I've ever played, and one of the best of 2010. It's completely insane. Honestly, I'd be judging you a little bit if you didn't pick it up.
Included in the Indie Royale Graduation Bundle is the greatly overlooked Russian horror game The Void, which I highly recommend if you're willing to put up with a bit of an unruly difficulty curve. There's Dead Pixels, an action-RPG that plays like an old school beat-em-up, puzzle platformer 1000 Amps, straight up platformer LaserCat, and multiplayer espionage/murder game The Ship. Finally, you get beta access to the upcoming MOBA AirMech. You know, if you're into that sort of thing. Even if you're not, the pricing plan involved with these indie bundles can't be beat.