Obviously there's no wrong answer in this thread, but IMHO omnisphere takes the cake. It's the most versatile, great presets, very adaptable, and sounds fantastic. Not to mention that the sampled waveforms give it an extra edge. And with omnisphere 2 being able to import your own samples... It'll be a fantastic sampler, rompler, and synth.
It is expensive though, but I think it's worth it.