Oh man, did I have Underoos. I loved them when I was a kid. I remember they had two styles: The "costume" style, which featured a T-shirt that was a copy of the characters' costumes, and "character" style, which just featured a picture of the character on the front of the shirt.
By far, my favorite was the costume Underoos. I had:
Superman -blue shirt with S shield, and red underwear with yellow waistband
Batman -Gray shirt with yellow and black Bat insignia, and blue underwear with yellow waistband
Robin- Red shirt with (R) symbol, and green underwear with black waistband
Darth Vader- Black shirt with circuit printing, and black underwear
Boba Fett- Gray shirt with green "armor" printing, and green underwear.
Spider-man-blue shirt with red and black Spider-webbing, and blue underwear with red waistband
Sybiote Spider-man-Black shirt with white Spider on chest, black underwear with white waistband
I did also have a couple of Character style Underoos, of Chewbacca and Luke Skywalker.