M0 are the heaviest ship class and, prior to the Terran Conflict, were used only by Xenon and Kha'ak. They are used as a story element, where their purpose is to destroy planets - or rather to be stopped from doing that by the player.
The Xenon M0, also known as the Xenon BMS, appeared in X: Beyond the Frontier and X-Tension. In X3, its remains can be found in Xenon Sector 101.
The Kha'ak M0 appeared in X2 in the battle for Omicron Lyrae.
In Albion Prelude, the ATF Valhalla is, canonically, an M0 Mothership, combining the firepower of a M2+ destroyer with the force projection of a massive carrier.