I like the McDSP ML4000 a lot for my mastering limiter. I also use the BX Digital, if you want serious mastering M/S, essential. If I want a little saturation to make it sound a bit more analog, URS saturation. The Digi rack 7 band is also really nice if you need to tweak the overall mix eq, and you can use the bx digital for surgical eq.
You really don't need a whole bunch a plug-ins for mastering, just get good ones.
Ozone 4 is a really good choice as well. I have used it for quick mastering a bunch and it works pretty great.
