Riku. I love the blue-purple color, that the heart is more like the KH1 logo, and I always did like the CoM artwork used there for him. I like the pose used for Terra, too. He, Sora, Xion, and Ventus are probably my next favorites. There's another Sora one, too, I think--one where he has the CoM artwork and a gold background.
They're all pretty though other than Aqua and (to a lesser extent) Mickey. I've always hated that artwork of Aqua. Mickey looks fine, although they made the artwork of him a little too large in comparison to the rest of stained glass, imo. I love the stained glass artworks, although I wish they'd have done a little more with the original characters' glasses than they did. I love how intricate the Disney princess ones were, for example the Cinderella one having different dancing positions in each outer circle or Belle's that have different dishes. I wish they'd make a comeback in the beginning of the new saga.