**
I sit corrected and admit my error.
I dunno, considering how focused on his science Reed can be. IMHO he’s the type to forget to put it on ( [Dustin Hoffman]I am not wearing my underwear. Definitely, definitely not wearing my underwear.[/Dustin Hoffman]). In one of the Byrne issues(number escapes me) Sue and the rest throw Reed a surprise birthday party. He’s surprised because he’d genuinely forgotten that today was his birthday. Reed (non-spoiler) later realises that his birthday was part of the knowledge sucked out by an alien machine. But nobody else finds it strange that Reed would forget his own birthday.
Galactus and Heralds-
Silver Surfer, Nova, AirWalker Automaton, Destroyer-
The first two were permanently covered in metal skin. The second two were constructs animated by energy. None of the four needed underwear.
Firelord, Terrax, Morg, AirWalker 1-
Terrax, Morg and Airwalker 1 (When he died, AirWalker 2 was built as a substitute) all wore clothing. What they wore under it is anybody’s guess. I’m not certain whether Firelord is wearing a unitard or if that’s his skin.
Galactus- I have an issue of Hercules in which Galactus removes his helmet. He’s got black hair and Kirbyesque beard-prongs growing from the corners of his chin. The white U against black background is not some kind of eyepiece set in his mask. Unless the big G wears contacts, those are his natural eyes.
Sadly, Galactus does not remove the rest of the armor. The reader is left to wonder if the world devourer wears boxers or briefs.
The Tick-
When Paul the Samurai prepares to depart, the Tick discovers that his blue suit has pockets("Pockets?! I have pockets?!). In earlier issues Arthur had wondered if the Tick was even wearing a costume. The antenna and blue covering could have been his skin.
Arthur-
In an episode of the cartoon, the pants to his moth suit are stolen. We can definitely put Arthur in the underwear category.
Various women-
The White Queen/Emma Frost, The Black Queen/Selene, Shanna, Marina etc
Emma and Selene wore lingerie as their fighting clothes while holding their positions as members of the Inner Circle of The Hellfire Club. Shanna wore a fur bikini. Marina wore a green, one-piece swimsuit. Wearing just underwear, or a swimsuit, makes the answer obvious.