As the owner of four Aphex Expressors, a Compellor and a Dominator, I obviously like their gear! Of those, the Expressor is the true compressor, while the others are a little different.
The Compellor is a leveler and the Dominator is a limiter. If you are looking for a standard compression type unit, the Expressor has the typical controls of threshold, ratio, attack, release. Expressors are great, and if you are buying used you can look for the older unit, the 651, and know you are getting something as good as the newer model, which is 661. Either is going to be fine.
The PRO VLA is also a good budget choice, although I think the Expressor is a finer unit. I also own
a PRO VLA and enjoy using it. I'm sure you would find uses for it as well.
As far as DBX, I have a bunch of those as well, but I don't care for the budget DBX units very much. If you must go for their budget line, don't go below the 166A in my opinion. Better still is to find a used 160X or 160XT.
The 160A is the current version of that unit. also, There is an older version of the 166 series that is considered by many to be the best. It is simply called the "166".