Interesting question and a lot of great answers.  I would add that material matters when selecting a compressor.  A compressor that is great for vocals ... may not be great for all vocalists.  So any formulas go out the door when choosing an energy management tool.  One thing I suggest that folks do with compressors is to become familiar with the technology the tool is modeled after or 'claims' to be modeled after.  Each physical and digital tool has characteristics even what we might call 'ballistics' to be considered in their selection.  Below is a table I use in one of my classes on energy management, I believe it was based on the compressors available in Logic - but the table illustrates the comparative model technologies, and provides some guidance to their use case.  Most important is to become familiar with the predictable behavior of the tool and then confirm with your ears what it is doing.  This is the same thing with meters, they should confirm what you are hearing, not telling you what you are hearing.  

I use the PC76, Ca2a, PC4K bus comp a lot. Sometimes the PC4K channel comp.

The Ca2a is fantastic.

Bass - Ca2a, sometimes after 1176

Vocals - 1176 followed by Ca2a

Guitar - often the 1176, sometimes even before an amp SIM.

PC4K bus - nearly always on master

PC4K channel - when I can't get the sound I want from other compressors, this can often work. Not as easy to set up, IMHO, but sometimes just works on the right material.

I have other versions of these - Waves, softube etc, but find that I can get the sound/feel I want quicker with the included Cake ones.

Please excuse my writing style- I am on my phone..

True this
 
a curious query by you the OP,
 
like see
 
Yesterday, I was stuck at a stoplight,
And there was this cat
She wanted a pair of socks,
I told her “I isn’t got no socks.”
She said, “Doot, that does not compute.”
I said, “What you say?”
She said, “damn, I could use a compressor.”
I said, “Naw, you need to get it right the first time.”
She said, “fux too.”
Not sure if we understood each other,
We laughed heartily,
cause pry we did.
Laugh Together.
Can you hear the laughter?
 
JQS
 
(whatever)
 

Hmm..

On further thought:

I wonder if it is just the Pro Channel workflow that I enjoy.

Since getting Studio One I have been using the Waves plugs more, and have been perfectly happy with the results, but if/when I move back to Sonar, I imagine that I will fall back into using the Cake plugs again.

Perhaps it is just handy having them in the PC.

I also occasionally use Boz's +10DB comp - much like the 4k Channel - when nothing else seems to fit..

I also like the Concrete Limiter - great for bringing up ambience on drums, amongst other things - the bass switch is lovely, too.. I used to use it ALL THE TIME, but use it far less now. I opened up an old project the other day from when it was first released, and it was on almost every track. 😀

Adaptive limiter is very nice for mastering.

Not sure that I understand previous post.

Personally, I think my previous comment is very lame.  I apologize to the forum for posting inane ramblings.  What I meant to say really, was that I am trying to eliminate compressors in my mixes.  Bht, google translate did a very poor job with my French, so please pardon my French.
 
When I use a compressor, sometimes I just use the most convenient one, like in the pro channel, especially whem I am on my laptop, because all I have is cake supplied units.
 
When I want to wade through my plug in menu I really like Pro C, and Black 76,  Also. I really like to experiment, so I will try almost anything, and if it sounds bad to me I will switch it out.
 
I think what I mean is that, I choose the compressor to fit the song, so it could take me several iterations to find the one that sounds good to me.
 
Again, I apologize for the ridiculous post.
 
jqs
 

Almost forgot to list another of my favorite compressors - Vari Comp by Softube (as sold by Native Instruments). It's almost like an LA-2A in its "can't really go wrong"-ness, but you have control over the attack and release. I have no idea what goes on behind the scenes with the innards of this compressor, but it sounds bloody amazing on almost everything. Its "comp" mode is a very low ratio of 1.5:1 so it's very hard to screw your sound up with it really, while its "limit" mode has higher ratios which are program dependant. Sounds very "high end." 

I actually quite enjoyed the"ridiculous post" - no harm no foul 😎

I generally have a kind of intuitive approach to these things, as with most things creative. Partly that's because I only half know what I'm doing with regards to the engineering side of all this, and I'm more interested in making more music than I am in buckling down and properly learning the technical stuff, so I kind of play to my strengths which are on the musical / creative side. Maybe that's lazy and I should spend more time shoring up my weaknesses, but I'm in this to have fun and it's more fun, for me, to just kind of dump the blocks on the floor and start making stuff.

So when it comes to compression I'm likely to just pick something that feels right, maybe something new I want to play with or something I haven't used in a while, and try it out and mess with the knobs a bit and see if it makes things sound better. Sometimes it does. Sometimes it doesn't but I learn something in the process, or hear something in a new way, that sparks a new idea.

That may not be helpful to someone trying to use the 'right tool for the job', but given how much beautiful and important art has been made over the centuries by breaking rules and misusing tools, I think a reasonable case can be made for my more intuitive, fun-driven approach...

The right compressor is the one you have that makes it sound better, not the one you don't have that you think will magically provide something you think is missing. Make some music with what you have.

He said, talking mostly to himself...

Jesse Screed
  Also. I really like to experiment, so I will try almost anything, and if it sounds bad to me I will switch it out.
 
I think what I mean is that, I choose the compressor to fit the song, so it could take me several iterations to find the one that sounds good to me.
 

The Klanghelm MJUC appeals to me for this reason, being able to switch between three different compressors in one and keep the same settings really appeals. To learn what it is that makes it sound right.  I just use the ProChannel options including the Boz +10, but the MJUC was going to be my next compressor purchase, but then the Scheps Omni deal came up, which also has 3 character compressors with the same settings to switch between am compare. Hopefully I'll start honing my ears so my choices are a bit more controlled and objective than they are at the moment

I usually have the MJUC in the master buss(very lightly), Softube FET on the guitar buss, CA-2A on vocals, Boz +10 on bass, and Kramer Master Tape on the drum buss.

LJB
The Prochannel comps are all really good IMO. PC76 on just about anything.

I second that! Pro Channel modules is basically all I need to get a great sounding mix. I tend to stick with just one set of plugins per mix (like just API or only Neve series by Waves), yet most of the time it's Pro Channel ones.