This is a tough one for sure. From my experience for the Virtual guitar emulators, Guitar Rig from Native Instruments has been the best for me. It really helps having a good solid preamp going to it though. That is a key ingredient that I think a lot of people overlook. I use it quite a bit though to re-amp if I do not want a real tube amp or am looking for a quick fix.
The BIG winner in guitar emulations to me comes from the hardware unit called the Fractal Audio AXE-FX2. That this is just unstoppable but you will spend a lot of money for it (2400 to 2600 street) but it is pretty damn amazing as to what it can do.