What specifically are you looking for?
I like the Waves Gold Bundle a lot, and some people are raving about the L2 in the Waves Mastering Edition (I haven't personally used that myself yet).
Other people seem to like Ozone, although I personally prefer the Waves stuff.
I guess a lot boils down to taste and pocketbook. But the C4 and L1 from Waves Gold Bundle are indespensible to me when mastering.
(BTW, all the above are DirectX. Can Cubase use DirectX plugins? Sound Forge can, I know.)