I am sure these are all different between them in more than one way, but definitely the most interesting will be the multitimbral VSTs.
That's why, I believe the sorts of Kontakt, HalionSonic or Sampletank will be most interesting.
IK's Sampletank 3 is going to be announced in January at NAMM, and it seems there are a lot of improvements and the interest builds up nicely.
Maybe vArranger could be a best match for such a virtual sound module! That will be killer.
Those would work great to build a base library for a software Arranger... I am a big fan of Kontakt myself, which works good when i trigger VST sounds from my keyboard. But these are merely advanced sample players
However, to get the true depth of VST�s you should also use some of the more specialised VSt�s that operate like true virtual instument simulations
But then i never realy use VST�s with my audia styles, i only use them as solo voices to play on top of those styles..