The best ones I have had are this FEA DB-CL, T-Rex Squeezer tube compressor, Akai Hexacomp (old school one with all the sliders), and I have played through a rackmount DBX 160A.
The DBX was awesome, but the FEA labs one is just as good, both very transparent. However the FEA has more tweakabiity than the DBX. And you don't have to rackmount it...