DKCR
Because it's the best platformer since 4th gen. Great gameplay, excellent level design, lots of challanges and secrect, kept entertained foe more than 150 hours. Easily the best after Prime 1.

As for Prime 3 vs Prime 2, I've preferred Prime 3. Prime 1 had the perfect balance between action and exploration and had the most inspired level design and artistic design. In Prime 2 exploration was really too slow and almost annoying, it had lots of dead times, battles agaist normal enemies (not bosses) where too long. On the other hand Prime 3 is more linear and less abitiuos, but still it has a better pace and it's generally better executed imo.

I'm torn between Prime 3 and DKCR.

Prime 3 controls like a dream, has some of the most compelling worlds of any game I've ever played, (Skytown, Phaaze, and the Pirate Homeworld are incredible) strikes an elegant balance of action and exploration/puzzles, and streamlines the pacing and navigation compared to Prime 1 and 2.

On the other hand, DKCR, is, in my view, one of the best 2D platformers ever made, with a wonderful sense of energy and verve, stiff but fair challenge, and exquisite level design.

Prime 2's still a very good game, but it had a few annoyances, (difficulty spikes, ammo for beams) that keeps it from reaching the same sky high tier as Retro's other games.