If you like shooters, I recommend Gunstar Heroes and Contra Hard Corps for genesis, as well as Super Contra for SNES.

The Mario games are classics as well, which for SNES would be World, All-Stars and Yoshi's Island.

On the RPG front, Genesis has the brilliant Shining Force series, while SNES has a host of offerings - Final Fantasy, Secret of Mana, Secret of Evermore etc.