My top picks so far:
• If you need something cheap, the best pedals in my opinion are the DOD Milk Box, Boss LMB-3, and Line 6 Constrictor, 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 Biyang CO-8, Moen Uni-Comp, 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 is excellent, and the BYOC Optical kit is a good value.
• If you want something a little better than those, but still not expensive, the Pigtronix Philosopher's Rock and Phil. Bass, Budda Chakra, 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, Dedalo Gorila, 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 Markbass Compressore, Durham Sex Drive, Carl Martin Comp-Limiter, OKKO Coca, Tortenmann TK-23, all of FEA's models, Taurus Tux, Joemeek FloorQ, Origin Cali76, and Menatone J.A.C 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, 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 models, and Effectrode.
• Limiters: the Rane DC24, Valley People Dynamite, Aphex 661/651, Keeley Bassist, Maxon CP9 Pro+, Pigtronix Philosopher series, Alesis MicroLimiter, Guyatone BL2, Boss LMB-3, various Joemeek models, Markbass, Demeter Compulator, FEA Comp-Limiter and DE-CL, Menatone J.A.C, Ashly DPX/CLX/CL series, Aguilar TLC, and Symetrix 501 all do a pretty good job of cleanly limiting big signal peaks.