The Aneros, every man should own one. I've been looking at the Nexus Ridge Rider for a long time but the price is holding me back on this one and to tell you anything about it. Don't know much about the others.
I don't rate non-vibrating prostate massagers very highly, so I'd discount the Aneros. Of the other three the Nexus Ridge Rider would be the one I'd pick because of its shape, and the position of the vibrating motor.
the ridge rider. i just got an aneros and while it can be used simply as a p-spot stimulator its directions for its "full" use are very complicated and it has a very steep learning curve (see their website and associated wiki pages)