The 600 series, the first-generation EOS, are great cameras, though the user interface is a bit odd. Their Achilles Heel is a shutter bumper that oozes goo over the shutter blades and needs to be professionally attended to. At one time I had quite an assortment of 600 series bodies, but have pared it down to three RT.
The Rebel series are the Entry Level models. They vary from 'no features at all' to full functionality. Small and light. No idea how quiet they are, and I've never heard anything against their reliability. Some have plastic lens mounts, and pentamirrors instead of pentaprisms, giving darker viewfinders.
The Elan series (except the 7N/7NE), plus the A2/A2E/5 and 10S, all have finicky mode dials that will eventually need replacing. The A2/A2E/5 is reputed to be a fine camera otherwise. A step above the Rebel, not of the EOS 1 class, but perfectly serviceable cameras.
The Elan 7N/7NE are pretty recent, and as such there are plenty of nearly unused ones available. No issues of any kind that I'm aware of. Very light and extremely quiet. Full functionality. Highly recommended.
The EOS 1 series are all heavyweights and, to my knowledge, have never had reliability problems of any kind. They tend to be loud. The best buy among this group, considering age, features, and price, is probably the 1N. Don't believe what people say about its weather sealing though; I took mine out during a very light sprinkle, got a few drops of water on the top deck, and the next day it was malfunctioning. A few weeks in a bag of rice dried it out, and is as good as ever. The 1V is a fabulous camera, but still commands a fabulous price. The original EOS 1 is a bit whiny.
My favorites are the RT and 1V, but I recommend the 1N and Elan 7N as being very nice fully capable cameras at a very good price.