Well now that you did… I hated that skull joke thing at first (alright, I hated Brook (and Robin (and Nami (sheesh, isn't there anything I liked in the series to begin with? I might have to start my career as a doujinka and rewrite the whole series if I keep this up))) in general at first), but now that I've heard it too many times, I actually find it amusing. Not necessarily my favorite joke, but amusing.
About favorites, one of mine was probably the intese stare of that flower-doctor-guy living inside Laboon. Not least because the whole scene was so… f'd up, or something.
Mystery things are cool, of course.