Most of the Japanese Megadrive releases from American companies such as Acclaim, Tengen and Virgin are very rare.
This is for two reasons. First, the the Megadrive was well eclipsed by the PC Engine at the time. Secondly, since American games are often largely ignored by the Japanese market, large print runs were not justified.
Some of the rarest rarest Japanese MD games are common US releases such as Maximum Carnage and Comix Zone.
Virtual Bart is up there too.