Currently am playing my way through Donkey Kong Country Returns. And I must say that I am extremely satisfied. It has to be one of the hardest plat-formers I ever played, or possibly the hardest. Which is great since I always loved the difficulty of the original DKC games.
I'm starting world 5 now, and can say that the fourth world kicked my ass many times. I probably lost about 40 lives between the mine cart and barrel blast level alone.
I highly recommend this game to anyone who enjoys plat-formers!