I think "best" depends on what you're looking for. I've got rather subtle/clean compressors in my Bassman 200 amp and Fishman Platinum Pro EQ Bass, but their effect is rather limited.
I've also got a Digitech "Bass Squeeze" that can be turned-up to have a MASSIVE effect (or turned-down to have a less noticeable effect).
Right now, I'm actually favoring the Digitech (unlike a week ago...), but "compression" in a stompbox really means different things to different people. Mostly, I'm looking to fatten my sound, for which a compressor really only plays a part.
A "real" rack-mount studio compressor generally has separate control over levels, ratio, threshold, attack, soft/hard-knee and perhaps other parameters. A stompbox without separate knobs for these things will by definition have to be limited.