My favorite is still the original Kirby's Dream Land. I played that game a lot as a kid and used to beat it without taking a hit. But sadly time has taken that ability from me. Star Stacker and Pinball are both super fun though, even though they're spin offs.
I'm surprised no one has said this, but mine would have to be The Crystal Shards - solely for being able to have two copy abilities at the same time. If I still played it, I'd still be experimenting with them, it was that fun.