I pretty much echo the Cakewalk plug ins. Getting the mix right (to my silly ears anyway) first. While I really believe most of what I go for are Platinum tools first, my extra plugin toolbox sets are...
Waves. I started with one of the basic sets, then I just kept buying them individually when 1 - I wanted to try something different and 2 - had a few dollars to spend. Started with the maximizers through L3-16 then went to bass enhancement, then vocal enhancement, then verbs, then liner phase tools (before we had the two sweet LP tools). The one tool I use the most is that C6 - it finds its way into spots where I can't seem to EQ well enough, which is about every time I work on trying to get bass and bass drum to fit in the mix.. I believe I have everything I want for now. Little bit at a time, when I was ready and had time to learn. Especially around the winter holiday season, they had some pretty nice deals.
Next up for purchase is Ozone 7 advanced. I like the idea that I can use the effects on channels AND when mastering, I can 'hear' through codecs. There's more in there, those kind of come to the top for me.
An honorable mention goes to all the freeware out there. MeldaProductions comes to mind formost. Their audio visualization tools like Manalyzer, MLoudnessAnalyzer, MStereoScope find a spot on the Master buss almost every mix.
Current wallet outgo is for a different kind of plugin - mics (you gotta plug them in don't you??). :o) It wasn't until I tried a few really good mics that I found what I was trying to tweek for, but could never seem to achieve.
Come to think of it - I also find myself reaching for the MOTU hardware plugin stuff too. Its actually quite good.
Take care
LL