I think it's hilarious now but, when I saw this thread title and saw Supergirl as an option, the first thing I thought of was: "If this is Helen Slater in 1984 wearing the Supergirl costume, she wins." Then I realized: "Oh, he means Tyler Hoechlin in the Supergirl television show wearing the Superman costume. Never mind." Hey, I was sleepy.
"Best" is subjective.
I personally like the George Reeves costume but materials have come a ways since then.
Christopher Reeve had the big budget classic costume which is my next favorite. I must not be paying much attention because I didn't notice that the Superboy costume was any different, at least in the first season which is the only one I've seen so far.
Next I would say the DCEU costume as worn in JL. It's nice and shiny.
Then the costume from Lois and Clark.
Then the Kirk Alyn costume although it's hard to say because the episodes of the serials are in black and white.
After that, the SUPERMAN costume from "Supergirl".
And last and least, the Superman Returns costume.