I vote for the Aneros products. The best is the original MGX, primarily because of the ridges at the stem that stimulate the sphincter muscles.
The Nexus line is suppsed to be great, too, but I can't speak from personal experience on that one.
The Aneros' are really fantastic. Comparing them to a butt plug or dildo is not possible. After using the Aneros butt plugs and dildoes felt like a I had simply stuck a piece of hard rubber up by butt!!
Of course, the best massager is attached to a man's body. Dick, finger, tongue - the real stuff is best!