My go to wides are the 24mm and 28mm primes, the range is very nice and easy to use and nice and fast f2/f2.8. I also have had a lot of use out of a 19-35mm tokina, its a very nice range though not as fast F3.5-4.5. And recently I picked up a sigma 14mm f3.5, its ridiculously wide for a rectilinear.

Much easier to use something that is very wide on a SLR than a rangefinder too.