Kirby Superstar is still the masterclass of Kirby. So I'll go with that. Wasn't as big on Forgotten Land as everyone else is. I know Kirby games are supposed to be easy but Forgotten Land's added Z dimension makes it almost impossible to lose. Nintendo also supposedly kept hammering HAL to include more enemies or to add some sort of added difficulty but HAL didn't budge.
The game is gorgeous and the music, especially in Wondaria, is amazing.
Return to Dreamland is also a favorite. I'm still more of a fan of 2D Kirby so I hope HAL continues to make 2D and 3D Kirby games like Nintendo still at least releases Topdown and 3D Zelda games.
Edit: Thanks for the image btw, it's now my new Desktop wallpaper.