I just got a Maxon CP-9 Pro+, which has the actual dbx compressor circuit in it, and it is really nice. It has 3 knobs - Threshold, Ratio & Gain. So you have a lot of flexibility for setting it according to your needs/tastes. Very transparent and quiet.
I also like the Line6 Constrictor, which models the Ross, Orange and an Opto compressor.