The compressor has become more and more a part of my sound as the years have gone by, after years of slow learning. Compressors have varied applications: some people use it as a pronounced squash effect, and others more subtly. Some use it as a boost. Some leave it always on, and some don't like paired with certain pedals.
In my opinion, you actually need to spend a lot of time using compressors to really understand how they apply to your sound and rig. My recommendation for someone new to the effect would not be to go out and buy the best, or most expensive, or most vintage unit until you are comfortable with the effect. You might find you like a certain type of comp over another - you might find that you don't need it at all.
It also takes time to learn the different compression controls (ratio, attack, release, input/threshold), and how they relate to the controls available on the pedal.
After years of playing compressors through my own rig, I had a clearer idea of the things that I needed from one. For me, this was:
Feature #1: EQ Transparency - Obviously, this relates to how the pedal affects your overall EQ. I would prefer for my comp to have a flat response, but some good comps I've played actually have a boost in the highs that somehow works quite well.
Feature #2: Noise - as low as possible, obviously. Noise levels will be depend on your settings. So, given your compression needs, does comp A have a lower noise floor than comp B...that would be my comparison. Every comp will amplify any noise that is in your signal already though.
Feature #3: Smoothness of Clamp and Release - I do not enjoy hearing or noticing the compression kicking in or releasing. Obviously this is closely related to how you set up a compressor. If I can find something that has enough compression for my needs but is still able to hide its work in the background that would be perfect.
Application #1: Low Volume cleans - I use compression a lot in this scenario: single coil guitar, no gain pedals, low overall volume. Single coils IMO benefit from the compression much more than humbuckers. Humbuckers seem to have a more smooth/level overall volume (natural compression? I'm not sure why).
Also, with my tube amps that are not being driven hard enough, compression helps with the feel of the notes, as if the amp was being driven more. If a compressor performs well in this application, it goes up a notch in my list.
Application #2a: To smooth out legato and/or tapping - I tend to use this for more high or mid gain sounds, or anything with a lot of legato and or tapping. It would be a low to mid ratio, low threshold settings.
Application #2b: As a boost into a gain pedal - A secondary application is when using single coils I sometimes like to set it up so that it is boosting the signal into the gain pedal up to the level a humbucker would be feeding into it. Settings wise this would be a boost in the compressor's output level control, with a compression setting of mid threshold, low ratio.
Application #3: Aggressive Rhythm Parts - I don't play a lot of funk rhythms but when the need arises, I tend to stick a compressor in there as the first pedal in the chain. It would be fairly aggressive compression, just to smooth out the volume of the ringing notes as well as percussive attack (think high ratio and low to mid threshold for a semi squashed feel).
That, in a nutshell, is what I look for in a compressor...so if I'm ever trying one out, I might plug it in and see how it sounds in Application 1, 2 and 3, and then also notice how transparent and noisy it can get in those applications. That's it! Of course, as my tastes change my needs will too. I might find more applications that become more important to me..that's part of the journey. Now on to the lineup I have at my disposal....
The Pedals
Boss CS-2
This little unit from the 80s is well known to many. I got this years ago and have been using it consistently for a long period of time. I am a fan, it does the job well enough. Controls for Level, Attack and Sustain.
Biyang Baby Boom Compressor X
This is a Ross style clone from Biyang (maker of cheap pedals in Asia). I got this just to see what the fuss was with Ross clones, and it was cheap. Word of warning, it arrived with a faulty switch. I rewired it myself and got a discount from the seller but if you are not DIY-inclined it is probably more effort for you than it is worth, if you get a faulty unit. 2 knobs for Level and Sensitivity.
Suhr Koji Comp
I happen to think highly of Suhr pedals. I really like their FX Link and buffered/true bypass options, and I find their pedals to be well thought out and the effects to be of generally a high quality. Controls for Comp, Level, Attack and Mix. There is also a voicing switch for affecting overall EQ.
Earthquaker Devices Warden
Newest in my lineup. Optical compressor with no mix knob like the Koji, however it does have more control over the compression in general with knobs for Sustain (input gain), Ratio, Attack and Release. There is no Threshold control - this can be affected/achieved with the input level, or Sustain knob.
The Shootout
For the evaluation, I'm going to be ranking them against each other in each category. I am not going to go with the star rating system, because to be honest it is hard to give a general rating with these things. It is much easier to rank them against each other in terms of how they perform.
Transparent EQ
Suhr Koji
Earthquaker Warden
Boss CS-2
Biyang Comp X
The Koji just edges out the Warden here, even though it doesn't have a tone knob. In the neutral voicing setting it does accentuate the highs slightly, but to my ears it leaves the everything else intact. With the Warden it's not hard to get something close to your original sound with that tone knob, I just found it easier with the Koji. The Boss leaves the sound slightly darker, as does the Biyang.
Noise
Earthquaker Warden
Suhr Koji
Boss CS-2
Biyang Comp X
The higher quality components in both the Warden and the Koji could be what is making the difference here. There is still added noise - none of these have perfect noise-free operation even within practical settings. But all are acceptable, even the Biyang.
Clamp/Release Smoothness
Earthquaker Warden
Boss CS-2
Suhr Koji
Biyang Comp X
The Warden and CS-2 both perform well here, with the Warden taking top spot because sometimes I have to look down to see if it's actually on! As with a lot of these judging criteria, the settings and your instrument level will play a big part. Choppiness or pumping can be a result of the Attack control.
Ease of Use
Earthquaker Warden
Suhr Koji
Boss CS-2
Biyang Comp X
For this category, I'm not judging it based on how easy it is to understand/use - if that were the case the Biyang would be on top with only 2 knobs. I've ranked them on how easy it is to get the effect that I'm after (see applications/uses above). Because of this, versatility comes into play as well, as a pedal would need to work for all the applications that I need it to work in.
The Mix knob on the Suhr puts it high on the list as I found it to be very useful in getting the sounds I was after. However, in the end, the level of control that the Warden offered just pushed it to the top of the list. It is more subtle than the rest of the compressors on this list though.
The Boss and Biyang are both fine comps - I would still use the Boss if I did not have the Suhr or Koji. I would only use the Biyang if I'm after a particular, squashed sound. It finds itself last on this list because of the limited applications.
Final Thoughts
The Warden is my current favourite and lives on my board just because of the versatility that it offers. It is not the "best" out of all these comps for ALL applications - for example, I like the Suhr better going into a dirt pedal and it the Biyang for that typical squashed sound - but it is by far the most versatile and does a more than adequate job in all applications.
I like the Koji's Mix knob, and its relay based footswitch. It's noise levels are low and the FX link functionality comes in handy if you are working with a system similar to the Boss ES-8 I use.
The CS-2 is an old and predictable friend and that is what it has going for it. It does have noticeably higher noise levels than the Suhr and the Warden but in almost all applications this is negligible. The Biyang is in my opinion, a good Dyna Comp style clone and does well in those situations where you need that sound - and that's it.