Kirby's Adventure: Nightmare in Dreamland for the NES is my favorite entry. The sprites are excellent, I love the overworld between levels, the music is top notch, and it's one of the few NES games with an autosave and progress meter.
Kirby Superstar for the SNES revisited the game, but it dropped the overworld, the music isn't quite as good, and the powers don't really feel right. They did include some newer abilities and co-op, which is nice, but it feels more disjointed than the original.