What fluid is the current recommendation?

The last time I built a car (GTD40) I remember a lot of "discussion" over DOT numbers - some being more/less hygroscopic, higher boiling point etc. I remember that the DOT relates to boiling point, so I was using 5.1 because I tracked the car a lot, and it seemed the best at the time.

But I've heard since then that it was a bunch of baloney, and that DOT 4 was just as good than 5.1, and better in some ways (like compressibility).

What does everyone use today?