Leadfoot
I agree with John. The Concrete Limiter and the CA-2A are excellent. I have the Softube Mix Bundle as well, and it is also very cool. I LOVE the TSAR-1 Reverb, and the FET Compressor sounds great on vocals and guitars. The Focusing Equalizer is pretty cool for bringing out the character of an instrument. There's also a Passive and Active Equalizer, and they sound good as well. I'm not gonna say what you should buy, but I do use the Softube Bundle, the Concrete Limiter, and the CA-2A on every song.
I echo that. Not sure which to recommend as I use them all regularly. I would say that I really, really like and use the Softube bundle and it's definitely more of a rarity to be able to buy it at a discount. So maybe if you think long term you may be able to stretch and get them all as time goes on, grab the one (Softube) that is rarer in terms of discount opportunity (which is now at the moment).
Also, want to put in a huge good word for the VKFX bundle. It's awesome! And a great Swiss Army knife of tools for the ProChannel. It sounds fantastic when racked up, which you can only get with the bundle.
A great video on the VKFX is here (the video doesn't show the ProChannel but it's there in the bundle):
http://www.youtube.com/watch?v=GSf-dJyzCVE