There are three basic options. If you want to to stick with normal quake the best option is DarkPlaces. It's highly advanced, with real time lighting etc, and has several mods designed specifically for it. The network code is also very good.
If you want to go with QuakeWorld then the best choice is probably FTEQW. It has the great qw netcode, as well as most of the functions of DarkPlaces. It can also be used to play Quake 2 and Hexen 2, with Half Life support maybe coming soon...
If your PC is a little older you might want a less intensive engine, such as Tremor. It still looks pretty tasty but is designed for lower end PCs.
The choice, as they say, is yours.