I do have favourite costume designs but I’d need to qualify my choices with some aspect of either character type or illustrative style. As a very young reader I thought the Superboy and Supergirl costumes looked great but didn’t like it so much on Superman. It was only in adulthood that I could understand my choice was (and still is) influenced by an illustrative style or the physical stature and posture of the character/wearer.
Daredevil’s costume is one of the few that survives a poor illustrator but Wally Wood’s version somehow seems best. Possibly because it was the version I saw first.
Other favourites, in no particular order. The Flash (on a skinny physique), Green Lantern, Atom, Adam Warlock, Captain Marvel (all best on ripped Gil Kane physiques). Doctor Strange—absolutely unique, like so many Ditko designs (although some haven’t aged well... like The Creeper. A truly novel design but clearly from another era) Captain Atom-- the original gold and red version—is another that holds up very well.
Jack Kirby has undoubtedly designed far more than any other single creator— as I kid I was amazed by Klaw, Psycho Man, the Rigelians, the Skrulls... (do aliens count?) But as some designs often varied from panel to panel, I prefer those rationalised by John Byrne. Like Galactus, Annihilus, Diablo. Whereas others I prefer the Neal Adams rationalisation. Gorgon, Karnak, the Sentinels. John Byrne is obviously also a great designer in his own right. Dark Phoenix and Silver Banshee both have an elegant simplicity I love. Whereas Neal Adam’s Havok is simple and absolutely unique.
I must also mention Dave Cockrum for his innovation. Nightcrawler in particular because of my long association with him.
I’m sure there are many, many more like the Doom Patrol (as a group??), Black Orchid, Spectre, Deadman, Eclipso, Black Widow, Iron Fist, The Vision, Yellowjacket,... to name a few off the top of my head.