It's been said a million times in this thread, but it's awfully hard to go wrong with the old 5D 'mk1.' I've had one for a couple years–picked it up for under $400, and it's been nothing but great. Back when I bought it, I wanted to get into a FF dslr as cheaply as possible. It was basically this or a D700, which was over $1300 or so at the time. I ditched my AF Nikon lenses and grabbed the 5D along with a Nikon F adapter form eBay.
Strong points for me:
-Pretty decent high ISO performance for such an old camera. 1600 works, 800 looks great. I never shoot film at 1600 anyway, so the lack of anything higher (the expanded ISO H––a sort of forced 3200––is acceptable in a pinch) doesn't bug me.
-Compatibility with a ridiculous amount of legacy lenses. Nikon F, M42, Exakta, etc. I didn't even have any native EF mount lenses when I bought the camera.
-Ability to use the fantastic Canon STM 40mm 2.8. Killer pancake lens for $130. My copy rarely ever leaves the camera.
-Surprisingly durable body. Mine looked like hell when I bought it, and it now looks even worse. It's not weather-sealed, but spent hours getting blasted with wave spray on a cold, rainy, windy day, and suffered no adverse effects.
-Great control and menu layout. Much better and simpler than a D700, I think.
-Nice raw files. Lots of room to play around with stuff, and the colors come out nicely.
Weak points:
-Manual focusing is so-so. I even have the better screen, and it still sucks on a 1.4 lens. I got an M42 adapter with focus confirm, spent half an hour doing micro focus adjustments, and now everything works much, much better. I'll probably get a Nikon adapter with the same chip.
-Apparently some type of mirror issue...it falls out or something? Mine has been making an unsavory sound in the cold for a while that seems associated with the mirror, but nothing bad has happened so far. Canon might still be warrantying it?
When this camera dies, I'll probably either buy another one, or an A7. I've used an A7 off and on too, but prefer the ergonomics of the 5D. I don't really have any desire to throw money at a 5d2 or 5d3.