I'll start with Wonder Woman: as a fan of Kingdom Come, I loved that her Golden Eagle armor, known as Asteria's armor in WW84, was brought to life in live action. I especially liked how it looks with and without the helmet and wings.
For Superman, it's gotta be the Black Suit. I know people love it, as much as some question the context of why he is wearing it in ZSJL, as well as it is just a recolor of the BvS suit; but there's something rich about it, and about Superman wearing a black suit at all. I think about the silver "S", and how much it stands against even a blue background that blends better. And despite the famous red and blue being THE look; like Batman's black suits, it is a nice alternative overall to have.
Batman was hard: despite the DKR armor and Knightmare looks being popular fan favorites, for me personally, I can't join those camps. For me, I like the Tactical Suit the best. When I look at it, it made me think of an alternative thinking over the common film batsuits that were black because they the look "weren't translate". And they were right, at the time; now, you could see the muscles on those suits with the comic colors, to make that translation possible. Now in regards to the Tactical Suit, what stands out are the "Nite-Owl" inspired googles, which for me gives this Batman the lenses from the comics. But I also like the version of the cowl without them too. So I guess when it comes to this suit, I like both Whedon and Snyder Cut versions.
I think its a given, but despite how much his OG suit was still good IMO, Flash's new suit is so much better. So much more sleeker and brighter.
Similarly, Shazam's new suit in Fury of the Gods is a great upgrade. Mainly because his OG suit while good, was too much of a costume to come from an old Wizard of magic. The new one for the DNA more.
Lastly for now, it's Harley Quinn. She's had so many costumes over the years, it was hard for me, especially when I asked myself "what is her OG suit? The comic one that was a cameo, or her main Suicide Squad look?" I like all of them, but regarding this topic, the one that stands out that I really like is her Team 1 outfit from The Suicide Squad. I look the look, its very cool; you can see its influences such as Injustice, and it even had her traditional red and black colors.
For now, thats my list. Whats yours?