From the "extra" PC modules, I guess the CA-2a is possibly my favourite/most used. As John T pointed out, it is great on vocals and bass.
The Concrete Limiter probably comes in second.
Having said that, the PC4K Channel compressor is a much underrated module - sometimes it is the only compressor for the job, but it just took me a little longer to get used to.
Unlike John T, I use the PC4K Expander/Gate quite a lot. I have it as the first module in many of my track templates. I like the way that having it and the channel compressor mimic the controls on an SSL console - not much visual feedback, but just dial it all in by ear.
I know the OP was asking about the "etra" PC modules, but just wanted to add how much I like the included modules in Sonar. I use the PC76 A LOT. It is truly great. The 4K Bus compressor is also really good - I use it sparingly on my master bus aaaall the time. Danni Danzi also turned me on to how good it is on a snare track, too. The Console Emulalions are also great, and used on most of my projects as well. Ditto the Sat Knob and Tube Sat module. Breverb (and probably REmatrix, when I have time to play around with it) is my go too reverb. Tape Sim is also good, but I find I use it less now for some reason - I use to be a big fan of Tape saturation on drums..
Of the third party stuff - the Overloud VKFX modules are also vey nice, as is the Softube Mix bundle.
So... You can probably tell that I like the ProChannel..
If I had to choose one extra, it would most likely be the CA-2a, but I would miss the others a hell of a lot.
Edited to add - I missed the out Quad Curve EQ, which is truly wonderful, and used on pretty much every track - with the Flyout and Analiser, it only got better and better.
Sorry - I will stop now..