My most favorite Kirby game is Air Ride, definitely. Heck, it's just my favorite game, period.
Least favorite would probably be either Avalanche or Epic Yarn. Avalanche for the characterization (Kirby being a total jerk,) and Epic Yarn for just not really feeling like a Kirby game.
Don't get me wrong, Epic Yarn is fun and has gorgeous music and art design. However, it's far too easy and it's just a little too obvious (to me, at least) that it started out as a game that had nothing to do with Kirby. For those of you who don't know, Kirby's Epic Yarn started out as Prince Fluff's Epic Yarn. Kirby characters were just added in because the main character, Prince Fluff, happened to already look quite a bit like Kirby.
It really says something for the series that none of the games have been bad. Sure we might complain about certain things, like the plot of Squeak Squad, the limited flight in The Crystal Shards, or the really bossy way that Kirby talked in Avalanche. But compare the least-liked games in the Kirby series to those of any other series that has over 10 games to its name. The Kirby series has gotten off really lucky, in my opinion.