Squidbaits Coupler Guide
OK, this is based on my experiences, and is strictly my opinion, so that and $1.50 will buy you a cup of coffee...
There are lots of proprietary couplers, but I'm just going to cover the major players.
N Scale
Rapido Couplers
These have been the standard in N-scale for years. Big, ugly, simple and reliable. They bear no resemblance to a scale coupler, have no magnetic uncoupling action (you need a raised ramp), or delayed action.
Pro's- cheap and reliable
Con's - big, ugly, no magnetic or delayed coupling action
MicroTrains
The gold-standard in N-scale couplers. Not scale size, but more realistic-looking that the Rapidos. Have magnetic and delayed uncoupling action. The inclusion of a small spring in the coupler ensures the knuckle closes automatically. Quite reliable, but fiddly to assemble and install.
Pro's - more realistic appearance, magnetic and delayed action, reliable
Con's - more expensive, tricky to assemble and install.
Accumates
The relative newcomer on the coupler scene, Accumates now come standard on many brands, including Atlas, Athearn and Roundhouse. Advantages are the magnetic coupling action and more-realistic size and knuckle appearance. Experiences vary in how well their uncoupling actually works. If you're using a pick, there's usually no problem, but they don't seem to be as smooth for magnetic operation as the MicroTrains, maybe because they don't rely on a spring to close the knuckle. Not as expensive as MT's either.
Pro's - inexpensive, good-looking
pretty reliable.
Con's - don't work as well as MicroTrains at magnetic uncoupling (IMHO).
HO Scale
The dreaded "Horn-Hook" or X2F coupler
This is the one that used to be standard on virtually all HO scale model trains. It was cheap, and the NMRA released it's patent on the design, so manufacturers didn't have to pay licensing fees to another company. What can I say about the horn-hook? It was cheap, and if all you wanted to do was couple, it worked. Uncoupling? Frustrating at best, since the original idea was you would run the cars over a raised ramp that would squeeze the pins together and uncouple the cars. The few versions of that that I tried never worked. Also, if you tried pushing a long string of cars, the action of the couplers would push to the outside of the cars, and if they didn't just uncouple, they would derail. Many people modified the couplers by cutting the dangling pins off, making a simple hook coupler, but really, they looked awful, and weren't particularly reliable.
Pro's - cheap like borschdt.
Con's - ugly, unreliable, difficult to uncouple.
Kadee Knuckle Couplers
Again, the gold-standard in HO scale couplers. Realistic-looking, reliable (if adjusted properly), and delayed uncoupling action. These couplers are sensitive to height differences, and the main problem people have with them is due to their not having set the coupler height properly. They will occasionally throw the knuckle-closing spring, which makes the knuckle not close on coupling. They are cast metal, and as such are very strong.
Pro's - realistic-looking, reliable, delayed uncoupling, magnetic uncoupling
Con's - more expensive than some others.
McHenry Couplers
This actually encompasses a few differently branded couplers, but they're essentially all the same. These come stock now on Bachmann, Athearn and other manufacter's models. These couplers (along with several others) became available back in the early '90s when Kadee's patent expired. They offered the advantages of magnetic knuckle couplers at a lower price than Kadee's. Of course, to do that, compromises had to be made. First to go was the material - these are made of plastic, so they don't have the strength and durability of Kadees. Second, rather than using a brass spring in the draft gear box to centre the coupler, they use whisker couplers on the end of the shank. These work well initially, but over time they break, or take a set, leaving the coupler uncentred, and unable to couple without some manual fiddling. Third, many of these type of couplers use a plastic leaf-spring molded to the shank to hold the knuckle closed. Over time, these springs will either break, or take a set, leaving the coupler unable to close its' knuckle, and difficult or impossible to stay coupled.
Pro's - cheap, reliable (for awhile), good-looking magnetic coupler
Con's - weaker construction, leaf springs break or take a set, impossible to repair, need replacement.
Accumates
These were Accurail's answer to the magnetic knuckle coupler. They are built along the lines of the MicroTrains N-scale coupler, a top and bottom half that slide over one another. They use plastic leaf springs to hold centre and close the knuckle. They don't appear quite as realistic as some of the other knuckle couplers, and (in my experience) aren't as reliable in operation. They have a tendency to drop their glad-hand (the magnetic trip-pin), and don't have a delayed uncoupling operation. They do stay coupled well, and couple easily, provided the centering springs are in good shape.
Pro's - cheap knuckle couplers, good coupling operation, fairly reliable.
Con's - unreliable magnetic uncoupling performance, plastic leaf springs take a set or break. Drop magnetic pins occasionally.
I think that covers the majority of the couplers in the two main scales... as you can see, I'm biased towards Kadee/Microtrains, but that's based on 10 years of club experience, with people using everything from horn-hooks through Mate-A-Matics (remember those!) to Kadees.
For reliability of operation and durability, I don't think you can do better than the Kadee metal HO couplers or the MicroTrains N-scale couplers.