I concur with what's already been said about the lens quality of the Autocords, and their build quality -- though the fragility of the focusing lever is definitely its Achilles heel.
Ergonomically it's not the best, though. The focusing lever for me doesn't come to hand as easily when looking through the finder as does a focusing knob (I mistake it for the shutter button) or the Diacord's "seesaw" levers, which are pretty nice to use. Also, I find setting the shutter speed and aperture is rather fiddly, and the settings can easily be changed inadvertently.
The Rolleicords are top quality in terms of build, though as mentioned a replacement focusing screen is really a good idea. The shutter charging and tripping lever is unique, but actually isn't bad at all -- it trips easily so minimizes shake at slower speeds. And I think they're the only ones of the knob wind TLRs (other than the Mamiyas) with double exposure prevention, which is pretty key. A drawback for me of these cameras is the way the shutter speed and aperture settings are locked together -- a common feature of '50s TLRs but a pain when you want to change settings.
The Diacords are really nice -- I'm looking right now at a G (unmetered, knob wind) that I just got back from repair and am eager to use. The Rikenon Tessar clone is quite good on these. No double exposure prevention, though, so you need to have a consistent shooting procedure in order to make sure you don't mess up.
The Yashicas -- seems like a lot of people sneer at them, but I have to say the Yashinon lenses are really very, very good. Not as good as the Xenar on the Rolleicords, and probably a little behind the Rokkor of the Autocords, but I've seen and taken some fine pictures with these. The 124 (not the 124G) might be my favorite of the whole series -- very good CdS meter, lever wind, and very sharp looking. I love the way the meter turns on when you open up the viewing hood. Frame spacing is sometimes an issue, but I think this is overstated. (About the only cameras with just about perfect spacing are the Rolleiflexes, it seems to me.) The wind lever sounds very rough with no film in the camera, but when loaded it's very smooth -- zzt-zzt.
One thing that's overlooked with TLRs is the aperture of the viewing lens, and here the Yashicas shine -- f2.8, with an excellent fresnel screen. Of all the cameras people have mentioned already, these are definitely the easiest to focus accurately in lower light.
And here's my "sleeper" -- the late model Yashica D, with the Yashinon lens (not the three element Yashikor on the earlier models). It has the f2.8 viewing lens and fresnel screen, and Rolleiflex-style focusing and aperture wheels, which are super easy to adjust. The only minus is that the camera has no double exposure prevention. These are underpriced -- I see them for US $100 and less all the time.