I honestly think it's UT99!
So simple, fast, modified to death with custom voicepacks, giant maps, mutators etc. - 8 years later and there are still hundreds of populated servers. Not many games can boast that.
Mainly it's the gameplay though, it's almost perfect, IMHO... Who cares about visuals? You don't notice textures when you're belting round a giant kitchen with the enhanced shock rifle trying to complete your m-m-m-m-monster kill before some sneaky sod sitting on a cupboard door ends your spree with a headshot!!