Donkey Kong County Returns. Played it back when I was younger and it didn't click with me. The platforming was too challenging and honestly I think I got too swept up in the hype around the game at the time.
Coming back to it later, I really appreciate it now. The music, animations and platforming is all sublime. It's sequel, Tropical Freeze is my favorite 2d platfomer hands down. It's an absolute joy to play