I tested out all of the Waves plugins and made specific notes to see if I really wanted to spend on any of them. In the end, I was particularly impressed by the API 2500 comp (used sparingly on a buss or 2, and used subtly on the master buss, produces great results) and the S1 stereo widener (which I use sparingly i.e. once or twice in a mix, typically on the FX buss as a way to widen reverbs, or if one track needs something a little extra)
I also use the 1973 EQ
constantly as my go-to. However, before, I tried countless free plugins and can say that Melda's MEqualizer is particularly impressive (especially considering it's free!)
I've also had audio engineer friends of mine recommend PSP's plugins. I haven't experimented with too many of them, but can say that the NobleQ Ex EQ is a great EQ that I consider one of my most-used EQs for mixing.