I have the Canon 100mm macro and 180mm macro. I've used the 50mm macro.
The 50mm is too short for a lot of macro use. To get close enough to the subject, the lens actually ends up shading the subject. If you are trying to shoot bugs, that is close enough to scare them off.
The 100mm is probably the most popular. It is a reasonably priced lens, and is more practical than the 50mm for most uses. It is relatively lightweight. It isn't an "L" lens, but it is a prime, and a good one. Lots of people also use this as a portrait lens. At this focal length, it has lots of uses besides macro.
I love my 180mm macro. It is a fantastic lens. Super sharp. Gives more working distance than the others. When I shoot real macro, this is what I use 90% of the time. That being said, I really don't use it all that often. Except for dedicated macro work, it spends most of its time sitting on a shelf collecting dust. It is a pretty expensive lens for limited use. It is also longer and heavier than the other lenses. It comes with a tripod collar. It is too long to use with on-camera flash. You need a ring flash or some off camera flash if you need extra light.
So if you are serious about macro, and you have money to spare, you can't go wrong with the 180mm macro. Fantastic lens. But most people settle for the 100mm, which is a very nice lens for a LOT less money.