Tough call between Rayman 1 and 2 for me.
Rayman 1's music is absolutely breathtaking in places, but the tracks are all very short and the majority of them are just ambient nature sounds.
Rayman 2 has the opposite problem, tracks are long and take far too much time to get to the good part, but when they do, man is it good.
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Rayman 3 has a few good atmospheric tracks and a few catchy, energetic tracks, but nothing really in-between. Doesn't always flow very well.
Origins took a lot of cues from both Rayman 1 and 2, some really short, orchestral tracks, and some tracks that take too long to get to the point.
Legends is a step up from Origins, it flows together a bit better but still doesn't have many stand out tracks.
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My vote goes to Rayman 2 since the main theme is my favorite track from the franchise, I get tingles every time I hear it. Some might call it overused, but it's masterfully woven into just the right areas of the game for maximum impact, and the choice to use it for both triumphant moments and in a darker tone for the pirates was absolutely brilliant. The overall use of the tune is the glue that holds the soundtrack together perfectly.