Pac-Man Championship Edition DX is something akin to pure fun. It follows up on the great rebirth of Pac-Man that the team at Namco introduced with 2007's Pac-Man Championship Edition. The game and its stages are designed around getting as many ghosts as possible, chasing you as you collect dots from side-to-side until eventually, the glorious power pill appears. Now, it's time to turn around and eat that entire chain of ghosts. Not only is it utterly satisfying to destroy your pursuers, but it has one of the best sound effects in gaming. Throw in an amazing slow-down mechanic when you're about to get hit by a ghost, bombs for bailing out of un-winnable situations, multiple modes, and leaderboards and you've got a near perfect retro game.
Runner-up: Donkey Kong Country Returns
Donkey Kong Country Returns marks the series' successful reintroduction and the passing of the torch from the original studio, Rare, to the new, Retro. While it isn't quite as difficult as the original game, it's hard enough to stand above the current crop of platformers as a challenging title that gradually introduces more mechanics, obstacles, and danger before combining all of them for the later stages of the game. The music alone should feed your nostalgia hunger and the game itself is a great mix of old and new-school design.
Yes, "pure fun" is the best description that I've seen of this game. It totally is like that. Well played.