Phantasy Star II was the best RPG of its generation. The seemingly simple game by Sega introduced complex, party-based combat to a generation of Genesis owners who missed the original Phantasy Star on the Sega Master System. PSII featured turn-based combat in which you could see your character carrying out the action you had ordered. This simple feature made combat extremely entertaining, and provided incentive to get new weapons and abilities. Watching Rolf, the protagonist, run up to a dragon and deliver a vicious slash with his laser sword is one of my favorite gaming memories.
In PSII you control a party of up to 4 characters. Players wander around a world map until they encounter one of the random battles that are extremely common. This game is combat heavy! Expect to spend most of your time wading through vicious battles and leveling your party.
While the combat is excellent, and holds up very well over time, the story is also extremely advanced for a Genesis game. PSII has a dark tone to it, and there is an excellent plot twist mid-way through the game. In some ways the plot mirrors that of Final Fantasy VII, despite the fact that PSII came out years earlier. I don't want to give anything away, but some aspects of those two games are VERY similar.
If you can get your hands on a copy of PSII (I'm not selling mine!) check it out. The graphics are surprisingly strong for a Genesis title, and the audio isn't bad either. Of course, this is one of my all-time favorites, so I'm a little biased.