Kirby and the Amazing Mirror. It had a great plot, huge levels, and it was a challenge for once (most Kirby games are way too easy for my tastes). It took me a while to get 100% (yes, I've done that).
I also like 64 and Squeak Squad. 64 was my first Kirby game, and I still like its large levels and good music. Squeak Squad, I didn't like it quite as much as Amazing Mirror, but it also had a good plot and it added great replay value to collect all of the treasures. Still, it was too easy, especially considering that Dark Nebula was easier than the Daroach fight before him!
I'm also fond of Tilt 'n' Tumble...Gah, I haven't played a Kirby game in a long time. I should play Amazing Mirror or Squeak Squad sometime.
I haven't played any Kirby game that came out before 64, except for Adventure on an emulator (and Nightmare in Dream Land). I just got Super Star on an emulator, though...let's see if it's as good as everyone says.