For a lot of capability and great results I really recommend iZotope's Alloy 2. Not expensive.

I have quite a few of the items mentioned above. Lots have great merits. I'd still recommend Alloy 2.

So many VSTs, so little time.

What version of X3 do you have?
Especially if you have Producer, I agree to Bitflipper. Take your time to study what comes with it before you start buying. The content list is impressive.
If you have the basic version X3, you might well get the best bang for the money uppgrading to Producer. I'm still on 8.5 so the X3-users surely can give better hints.

I'm on 8.5, too, which is one reason I have as many third-party plugins as I do. Prior to the X series, SONAR had three critical deficiencies that had to be addressed via third-party plugins: it lacked a decent limiter, the bundled delay was buggy, and the pitch-correction tool was tricky to use. These have all since been addressed.
 
For bread 'n butter effects, SONAR now lacks very little. Use of third-party plugins nowadays comes down to personal preferences and specialty effects. In the latter category are amp sims and distortion (including tape sims and saturation effects), sound-manglers such as vocoders and stutter effects, modulators such as choruses, autopanners and filters, and specialty dynamics processors such as gain-riders and transient boosters.
 
And, of course, reverbs. No other type of effect plugin is more subject to personal preferences. Perhaps the most-frequently recommended third-party plugin is ValhallaDSP's ValhallaRoom reverb - fifty bucks (!) and plays in the big leagues.