Its not about which ship has the better piano bar.. it's which player is there that makes the difference. Not all piano men are created equal!
Generally the newer ships have the better piano players (ron pass, brad alexander).. They take requests, sing their own songs, you drink, you tip and everyone has a good time.
I also understand that some of the "older" ships (same class as the imagination.. I forget which is which) have a bad layout for the piano bar. Its out of the way, so you don't get a lot of walk ins.