I've gotta agree with some replies though, I don't really care for the opening stage's theme that much. The soundtrack picked up quickly after with Through the Tunnel later in World 1 though, and kept up in quality throughout the whole game.
Another one of my favourites is Abandoned Beach, the first level from World 2. It was one of Jun Ishikawa's (long-time HAL composer, worked on Kirby since the beginning) contributions to this game, and it's great. A few of his other tracks have already been posted here, even (the aformentioned Through the Tunnel; and Burning, Churning Power Plant). Obviously this one has the Float Islands leitmotif, but it really hits those Aqua Star vibes too; he composed both and this is another beach level theme so that's only natural I guess.