Super Robot Retsuden

The ultimate cross over manga featuring Mazinger Z, Great Mazinger, Grendizer, Getter Robo, Getter Robo G and Jeeg! These all-stars band together in order to defend Earth from cosmic evil. This manga is in the same vein as all those old Toei cross over movies, and the self-parody gets pretty strong later on.

(Source: Dynamic Pro Scanlations)