So I'm still pretty new to DAWs and such, and haven't been able to invest a lot of money into it yet, so I've been sticking with free VSTs. My favorite so far has been Atlantis: http://jeremyevers.com/atlantis/.
It's a really awesome plugin with 3 oscillators, 2 filters, 2 envelopes for modulation, a sound shaper, 2 combiner modulators, and a pretty neat delay section (with it's own 2 filters). Pretty much every aspect of your sound can be modulated (volume, pan, wave width, phase, filter freq/thresholds/resonance, etc). The link also provides a nice guide on how to use it. Nearly every synth I used in the first remix war was using Atlantis, and both my songs for the current remix war use it as well.
Anyways, thought I'd contribute!