I just purchased the 4 Rayman games that GOG has available. I have never played anything from this series, so I'm hoping someone can help me find a good starting point.

GOG doesn't appear to have the original game in the series. Should I attempt to pick it up elsewhere? And where do the 2D games fit in?

Thank you!

It does, it's Rayman Forever.

As for the starting point, I would follow the release order. Although Michel Ancel (the creator of Rayman) wasn't involved with the development of Rayman 3, and as such those events are largely ignored.

I also agree with release order, but quick word of advice: You may want to look up the music fix for Rayman Forever.

Good luck with the games, they all get really hard(The point when that happens gets later with every game, though).