Well all 3 are different and they are all good. Volume 1 is a bit annoying because I don't really like Sarah Brightman's voice. I think Volume 2 is the one with the more enjoyable version of the main titles song by another singer. I haven't listened to all 3 in comparison enough to be able to give you a suggestion about the differences though.
The theme Hisaishi wrote for this is fantastic, one of my favorites. Right up there with Tenshi Meisatsu, which I also highly recommend.
Since Hisaishi builds his material from season to season (and album to album), make sure to eventually get all three. But indeed, #2 is the most entertaining and varied. The second track, "Gaki daisho! Masayuki" is one of the most catchiest things I've heard in a long time. And it comes with plenty of cowbell!