Most Olympus OM primes are still a good value, and are excellent performers on a Canon EOS body with an inexpensive adapter. The 18/3.5, 21/3.5, and 24/2.8 are well worth using. Ditto for the 35/3.5 PC shift lens. The 18 is getting tough to find, but they are available (but usually at a premium). Great lens, with little distortion. The longer teles, 200 and 300, are also others to consider (and very inexpensive)...you can get a 200 prime, with adapter, for under $100.
Most C/Y Zeiss primes are also excellent performers and are moderately priced. I use the 28/2.8 and 35/2.8. There's also the C/Y Zeiss 35-70/3.4 zoom...just about as sharp as any prime lens and also very moderately priced. Then there's the legendary Zeiss 21/2.8, but I'm passing on that one.
I'd say the easiest lenses to adapt, with pretty much zero possibility of any interference with the camera's mirror (5D and 5D2) would be the Olympus OM lenses. Very small, very discreet, and basically the same filter size across most, if not all, of the entire range. If you do any hiking, these lenses are so small and light that you could carry a full complement of them and not even notice them. This is why I have so many OM lenses in my gear list.