The two for the Playstation are remakes of the Sega CD two, so either set of those. The GBA one is a terrible remake of the first one, so avoid that if necessary. There's also a PSP remake of the first one, I haven't tried that yet.
There's also Magic School Lunar/Lunar Walking School, which was for the Game Gear and Saturn but never released in the US.
And then there's Lunar: Dragon Song which is uhhhhh and I've never played that.
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TheKnightOfNee