I've enjoyed all of the Kirby games I've played, but Kirby's Adventure Wii does hold a special place in my heart. I think it's a mixture of good soundtrack, some copy abilities I really liked, and it's one of my last gaming memories of playing cooperatively with my family.
Well, the only 2 Kirby games I've attentively played through so far are Kirby 64 and Air Ride.
I'm pretty excited to eventually play the 3ds kirbys.
For now I'll have to say Air Ride, I played many many hours of it, mostly goofing around in City Trial with friends, making up RP's basically, when I was a kid. I remember I also loved to RP by myself in the race courses, I really enjoyed the elements of the world and characters, and how with every lap the environment changed.