Dave picked up the banjo because of Pete Seeger and his playing style was very much influenced by Pete. That influence went to the extent of incorporating Pete's new invention, the long-neck, as an iconic visual symbol of the KT. But I often sensed that Dave's playing was sometimes on the verge of derailing. (I'll have to go listen for some examples.)
John also came to the banjo as a second instrument and I believe he tried to follow in Dave's footsteps. He continued to primarily utilize the long-neck, but, as with Dave, I often sensed some raggedness in his banjo playing.
George Grove is a more studied player and he plays with great precision. While he has thoroughly studied his predecessors' styles and on some tunes attempts to recreate those recorded banjo parts, he is more fluid than either Dave or John. I enjoyed working with George - he was usually spot on. EXCEPT - his banjo playing on my recording of "MTA Revisited" (on my "After All These Years" CD) is a mess. My producer tried to hide it in the mix, but if you listen closely you will note that his timing is off - most evident on his solo in the middle of the arrangement. (If you go to my home page here that song plays automatically.)
Bill Zorn is a totally different kind of banjo player. His style is very melodic, as opposed to chord-oriented. He uses a lot of chromatic runs in his playing and I don't really know who to compare him with. I certainly can't compare him to either Dave or John, or even George. And I don't think his style fits the KT particularly well.
These are all my opinions and you can take them for what they're worth - just my opinions. As far as preferences or rating, this is the best I can offer. I like Dave's playing because it was kind of loose and on the fly a lot. Being the original banjo player he set the standard, I guess, for KT banjo playing. I never cared for John's banjo playing, so I'll leave it at that. I think George has done the best job of utilizing the full possibilities of the instrument in the context the KT. Bill Zorn - good player but not in the KT style.