For me, I did amass a number of plugins, but what I did was re-evaluate the ones I simply use most:
Waves Kramer Tape
Waves SSL4K EQ
Waves PuigChild
Waves PuigTec EQs
Waves NLS (Mainly for the Mike Hedges emulation and channels 27 and 29 of the Spike SSL emulation)
Waves CLA-76 Blue and Black
Waves IR-1
Waves L3-16
Waves CLA Signature Drums and Effects (the former for the Soft-gate)
Waves API Bundle
EastWest Quantum Spaces
Flux Alchemist for Mastering fixes
DMG Audio EQuality and Equalibrium
Sonolksys CQ1 and DQ1
Algorithmix Red
Aside from these, I don't find I need many other 3rd-Party plugins. Unfortunately, for me, the Sonar-supplied plug-ins are so cramped I don't like to use them as much, but they are quite good. However, I love the SSL4K EQ from Waves quite a lot, and the CLA76 compressors from Waves. A good convolution reverb is miles beyond a local theoretical reverb, in my opinion. I don't like synthetic verbs for bands. I do wish AudioEase had Altiverb for Windows.
Some of the plug-ins I use for Mastering, but for Sonar, I would go with finding what you can afford and want in these categories:
1) Cleanup processing: DeEsser, Crackle/pop/buzz removal, phase-Correction
2) EQs: I like the larger interfaces, but I like when I have one that just sounds good to me for character, such as the SSL4K (which the Sonar EQ comes close to, but it's too small for me). Get a good surgical EQ, such as the DMG Audio EQuality or Equilibrium. Sonalksys has a great dynamic EQ (DQ1) which can allow you to fix an over-exaggerated kick drum.
3) A great convolution reverb. Waves has a discount going on Facebook for the IR-1 at $65!!
4) a great limiter: Waves L2 and/or Waves L3-16. Flux also has a great one.
5) Emulators: I love the Waves Kramer Tape. I'm really pressuring those guys for a new plug-in I think will be amazing and they can certainly do it, but no such luck yet. Console emulation is useful, but I don't say I absolutely HAVE to have it. I use it when I just want to do
something the audio of a track, but I'm not sure what. CE just adds some magic. I'm not buying into the whole collective use of CE yet. To me, CE is better suited to projects where articulation isn't as important as vibe, spirit, and emotion. So, i use it sparingly cranking the drive a little.
6) Imaging - Waves S1, for example, or the free Flux StereoTool.
7) Dynamics: I love Waves CLA Black and Blue. They are the best ones out there for me, because they are great at compression, but super fast attack and release, not to mention Waves includes the ALL button. However, the 76-type Compressor included with Sonar is awesome as well. Flux has a free BitterSweet tool to help retain some punch if there's a lack.
8) Metering - Most DAW's lack good metering. Blue Cat has some cool meters, as does Nugen Audio, Brainworks, Algorithmix (a semi-free one), etc. Waves also has a good one, but it's rather small (unless you get the Loudness plug-in. Flux has the expensive but amazing Pure Analyzer System. It's one of the best I've ever used, but it's way expensive.
Most everything else Producer has. Get a few and master those plug-ins. When they just can't get you what you want, try a few more, but don't hunt for plug-ins out of boredom, for sure. I agree with that inferred implication above.