I really like the distinct vibe they gave to each of the Robin costumes. Just by looking at them you can easily tell who wore what. As for the best looking, I'd have to say Grayson's costume looks coolest. Though I also like Damian's.
Is it a cop out that I like them all equally? I really can't choose, they are all great for their own merit, and I think it matches them. I mean I can go by which Robin I prefer (which is Tim) but that has nothing to do with their costume.