Erin Shore. I've heard versions by The Corrs (drum solo lasts way too long, but otherwise good) and The Royal Irish Regiment, which is alright, though each repetition lacks uniqueness, they're all very much the same to my ears. And the bagpipes just aren't as soothing as the instruments used in the Corrs' version.
Oh, also the version from a "Celtic Dreams" CD. It sounds like something out of a jazz club that uses poorly synthesized instrumentation.
The Wallace and Gromit theme is pretty catchy, too.
Other good ones:
Enya
Comb of the Winds
Sun In The Stream
To Go Beyond (II) (vocals, but no words)
Sarah McLachlan
Touch