See the Dampa mount thread below. My vote is the Beeman mount, it's built like a brick sh#thouse! Its windage and elevation adjustable, easy to zero, and has never moved on my 48. I tried the RWS c-mount (twice) and it just wasn't working. Just my 0.02!!!
I have used in the past the OLD RWS C mount with good success.
There are two aspects to its good usage:
1.- The stop screws need to be lathe turned into real dog-face screws. Meaning that the "point" of the screw is a cylinder that enters INTO the rail cavity and abuts perpendicularly. If you try to use the common screws provided by RWS, then the cone in the screw point actually drives the movement of the mount.
2.- It has to be perfectly degreased and mounted with the right torque and with blue loc-tite.
Once adjusted in this way, it stays there forever or till you move it. If you cannot perform BOTH these operations, then stick to BKL's.
IF you can afford'em, drooped BKL's are better, whether custom drooped by people like Mac-1 or the standard drooped by BKL themselves, but they are expensive in the sense that once made they stay with the rifle they were made for.