Now is a good time to pick up plugs, both for the quality that plugs achieve now, and for the value (lots of sales going on now). Just keep in mind that a mix engineer worth his weight will be perfectly capable of delivering a quality mix with stock plugins. So you don't "need" any of the following plugs.
+1 for Softube, SoundToys, SSL & L2 from Waves. I like RVox as well but I'm pretty much using Sonnox Oxford, SoundToys, & Softube the majority of the time. Waves are cool but HIGH and you can get a different variety of plugins at that price. I'd recommend Sonnox Oxford Elite!