Spectrasonics Trilogy.
Hands Down. I didn't believe the hype at first, but its one of the nicest VST's period, not just bass. (IMO)
EDIT: oh damn, i read ur post but somehow missed the besides trilogy,
Arturia Minimoog has some nice sounding synth basses, i've made quite a few bass patches for it. Thats about it, i usually focus on orchestral instruments in vst's.