There were a great number of fantastic 2D games released this generation, which was great to see after they were nearly dead in the PS2 era. I've bought quite a few and here are my favorites. I'm curious what everyone else thinks:
Muramasa: The Demon Blade (Wii)
Extremely polished fighting system and beautiful graphics. Might be neck and neck with Bioshock for my favorite art style this gen. The biggest problem is the level design and backtracking, but the allure of collecting all 108 swords in the game is a great incentive to keep playing after you've finished it.
A Boy and His Blob (Wii)
A great looking and fun remake of the NES classic. The levels are designed well and the use of the beans to get around puzzles is often clever and always fun.
New Super Mario Bros (Wii)
Not much to say about this one. It's classic SMB updated with more stuff to do, new powerups and better graphics. And multiplayer, which is a blast if you can get three friends to play along.
Super Paper Mario (Wii)
Not entirely 2D, but enough to list it here. I've always liked this series and it was fun to see it go in a new direction.
Rayman Origins (360)
A really well-designed platformer, with beautiful and colorful graphics and a weird sense of humor. Easy at first, but halfway through it becomes really hard to collect all of the goodies if you want to rank higher and open up more unlockables.
Deathsmiles (360)
A cute bullet-hell shooter that loves to punish you with screens full of bullets and enemies. It feels like a game that would be comfortable along the best shooters on the DC like Mars Matrix or Gunbird 2.
As a collector, I generally try to avoid downloading games, but there are just some games that wouldn't survive as a retail release that flourish on services like XBLA.
Shadow Complex (XBLA)
A great-looking and fun game heavily influenced by Metroid and Castlevania. Exploring the levels and acquiring new abilities is an absolute blast.
Cave Story (WiiWare)
Feels like a classic but forgotten 16-bit game. Strange characters and fun addictive gameplay make this a must-have for any 2D fan.