Secret of Mana, Final Fantasy VI, Earthbound, Chrono Trigger: that's my order from best to worse. Maybe Final Fantasy VI simply because it's more a 1 player experience..what about Final Fantasy IV though? You could play it on GBA remade...
However, picking up a Genesis and reliving memories recently, I'd say you find $50 and get a Genesis with Phantasy Star IV, Shining Force II, and Beyond Oasis...I got Beyond Oasis for $1.99...cartridge only copies of SFII and PSIV go for $15 plus shipping....best $50 you'll spend in your life...