My top picks so far:
• If you need something cheap, the best pedals in my opinion are the DOD Milk Box, Boss LMB-3, Moen Uni-Comp, and EHX Bass Preacher, pretty much in that order. If you want a rackmount unit, get the Alesis 3632. Others that are pretty good for the price include the Line6 Constrictor, Biyang CO-8, Alesis Nanocomp and Microlimiter, Ibanez BP5, Guyatone ST2, dbx 163X, Behringer DC9, and ART Levelar. And if you want a DIY project, the Madbean Afterlife and the Schalltechnik_04 Pumpernickel are excellent, and the BYOC Optical kit is a good value.
• If you want something a little better than those, but still not expensive, the Seymour Duncan Studio Bass and Vice Grip, Dedalo Gorila, Pigtronix Philosopher's Rock and Phil. Bass, Budda Chakra, recent-production Fromel Velvet Vice, and Carl Martin Classic Opto provide a lot of bang for your money.
• The top tier pedals that I'd recommend to most people combine ease of use with great tone and low noise: the Whirlwind OC Bass, Diamond BCP-1, MXR M87, Keeley GC-2, Empress, Maxon CP9Pro+, Effectrode PC-2A, Mad Prof Forest Green, G-Lab BC-1, Wampler Ego, Aguilar TLC, Earthquaker The Warden, and of course the OVNIFX Smoothie. If your tastes are more exotic or particular, the RMI Basswitch, Sonuus Voluum, Markbass Compressore, Origin Cali76 (all versions), Durham Sex Drive, Carl Martin Comp-Limiter, OKKO Coca, Tortenmann TK-23, all of FEA's models, Taurus Tux, Joemeek FloorQ, and Lightning Boy Op-2 are also very much worth your attention.
• For rackmount units the Rane DC24, dbx 160X/XT, Ashly CLX/DPX series, Symetrix 501, Aphex 661/651, and JBL 7110 are all excellent. They each have unique qualities that make it hard to pick one "best" one. On a tight bugdet, the new Alesis 3632 and the old dbx 166 are very good values. I am also very fond of the FMR PBC-6A, ADK CLA-1, and Joemeek MC-2, but they are half-rack modules, sometimes difficult to mount in a regular rack rig.
• Tube compressors: my favorites are the Effectrode PC-2A, Retrospec Squeezebox, and Markbass Compressore, though the Lightning Boy is worth your time too. Read this article for more info about tube comps.
• What do I use personally? Lately it's a three-way tie between the Origin Cali76 TX, the FEA DE-CL, and the Smoothie. Each has qualities that put them on top for different moods. Other personal favorites include the Empress, Markbass, G-Lab BC-1, Diamond BCP-1, Retrospec, Fairfield, ADK CLA-1, other FEA and Origin models, and Effectrode.
• Limiters: the Origin Cali76, Rane DC24, Valley People Dynamite, Aphex 661/651, Keeley Bassist, MXR M87, Maxon CP9 Pro+, Pigtronix Philosopher series, Alesis MicroLimiter, Guyatone BL2, Boss LMB-3, various Joemeek models, Markbass, FEA Comp-Limiter and DE-CL, Ashly DPX/CLX/CL series, Aguilar TLC, dbx 160A and its clones, and Symetrix 501 all do a pretty good job of cleanly limiting big signal peaks.