I think there are some hands-on disciplines that you could learn properly on your own, but which you could never be the best in the world unless you learned from a master.

Think woodworking, martial arts? Many more I'm sure.

I don't think this will be a very big list, though. You should be able to learn almost anything these days. I don't feel inhibited by anything except the sheer depth of some topics, and lack of my own time. I pretty much feel like I can learn anything.

Almost any topic can benefit from a good instructor, though, especially if it's one-on-one. They can steer you better than you can steer yourself as a newbie.

Is there a purpose behind this question beyond curiosity?