This might not get a ton of support, but I'll say the 1940 Play Ball cards. Sure, they are black and white and boring to many, but they were a mainstream set that we see features of in the card world today. They have a nice photograph and a simple and consistent border design which has ruled the card world. They also did this while cards were moving closer to the modern size. Think of all those sets from the 60s, 70s and 80s that have some sort of consistent border design and a nice image. It's even a good formula with color images.
Also, they were using some throwback players which has become popular even in modern cards. I think the Play Ball cards were way ahead of their time.