Ive had meguiars, mothers, and claymagics.
I recently bought one from Adams Polishes (along with their Quick Detailer) and it blows them all away. Not so much the ClayMagic one, as its a very good claybar as well. The Adams Polishes clay and ClayMagic are probably the same as far as results go, the adams one if just easier to use, and i like the container better. The megs and mothers bars are both kinda hard and dont really pick as much stuff out of the paint it seems like.
The detail spray is also the best smelling stuff Ive ever smelled. Im tempted to drink it everytime i use it.