Tiger (10.4) is a great system, and offers a lot more compatibility with modern applications. It's also a bit faster than Panther, given appropriate RAM amounts.
If your PB has less than 512, I'd say Panther is probably still a good option, but if it's got 512 or more, try Tiger. Tiger is amazing with 1GB or more, and all PB G4s can take at least 1GB.
I use my PowerBook with Leopard, and haven't been on Panther in some time now (few months, had it on an iBook). Tiger is a solid system and I think you'd enjoy it's benefits