I like the 4 Sevens Quark clips, both the normal and the deep carry. Not as big of a fan of the one on my (otherwise great) Surefire C2.
Yeah 4sevens is a good design for a clip-on with the screw down collar. It's pretty substantial. If I do have one compliant with it, it's that it comes so frigg'n tight and will rip up my pants pockets if put-on/taken-off single handed. I can try and bend it, but with clips, it always seems to be the same thing.. doesn't bend until it bends too much.