I personal like QuakeForge http://sourceforge.net/projects/quake and recommend it. It fixes allot of bugs and a native win32 version is available. It works allot better and allot of BSP bugs and such fixed.
~Open Arena~ A complete re-write of the Quake I Engine, its lean mean and damm sexy. Its 100% free and does not need a Quake CD. It only uses the Quake I Engine with lots of improvments and bug Fixes. Link: http://openarena.ws