It has to be Fuhquake.
It has lots of new eye-candy such as support for 24-bit textures, fancy exposions, skyboxes, etc. It has lots of new teamlplay features, it's fast, it's stable, and you can customise almost everything about it. It and mqwcl are the only two clients allowed in competitive play. It supports MVD playback. It supports MP3 playback via XMMS. It's well documented too.
Quakeworld.nu recently ended a client-related
poll and it showed 87% of members use Fuhquake of some kind.