OP was asking about a compressor for slap and I'm into that, so here's my .02:
I've used a Mark Bass Compressore, an FEA DE-CL, an FEA DB-CL, an EBS Multicomp, and an OvniFX Smoothie. I like 'em all, and still own all but the EBS, which I sold to fund a cab purchase a while back.
I've been gigging straight into the amp for the last year or so, but just today I was dinking around at home with putting together a micro pedal board--just a high pass filter, a strobe tuner, and a compressor...and maybe a VT bass pedal (still pondering that one for the more aggro tunes). Y'know, my fave for the compressor is the Smoothie. Especially when slapping, I really like having the dynamics available under my fingers, and the Smothie's ability to blend dry and wet signal made the difference. I can set it to rein in pops that are a bit over-enthusiastic, but I never feel like I'm fighting the compressor to get some notes to jump out when I want them to. I gotta figure this is because I have a good amount of dry signal making it to the amp.
Swearsies, I know how to work the controls on these things. But the Smoothie is the one with which I can get natural-sounding dynamics and just the right amount of control over transients. IMHO, YMMV, as always.