K&M 23720 3/8"

This clamp is awesome! The clamp itself is very sturdy and will stay put, but it is a bit bigger than other clamps, so it might not fit everywhere.
What sets it apart from other clamps is that it has four attachment points, which makes it _very_ versatile, and you can buy additional screws to use more than one attachment.
Combine it with a stereo bar or goosenecks and you can for instance mic a grand piano, a guitar cab or parts of a drumset using only the one clamp.

I work as a FOH sound tech and I'm very fond of clean stages, so this thing is a godsend for miking things without having to deal with mic stands (that always gets knocked around).