For me, I'd have to say 2e to 3e. Now, granted, I have no direct experience with OD&D to Supplement I, so, I cannot comment there. I really don't know.
But, going from 2e to 3e marks, for me, the division of game design from talented amateur to professional. Now, put down that tar and feathers and let me explain. No, really, let me explain. Ow, ow, hey! Ok, fine. Now let me explain.
3e was the first edition based at least in part (and I think large part) on market research. Prior to 3e, material was released into the wild with little knowledge of how well it would do, and how well it did after the fact, other than gross sales. It was the amateur approach of throwing things at the wall and seeing what sticks. Thus, game development tended to be very organic in nature. You had a huge number of supplementary material in 2e and the approach seemed to be very unguided as to what got sent out on the shelves.
3e changed all that. Lazer focus, very strong oversight on books and very high production qualities.
Could you imagine a book like, say, the original Unearthed Arcana coming out for 3e? With the warts included in that book? I don't think so. It would never get past the marketing department.