In the annals of truly whacktastical fantasy movie outfits, nothing can really beat Sean Connery's accordion suspender/granny panties/thigh-high boots combo in Zardoz. It's not fair to make any other, lesser ensemble try.
However, if I may humbly submit an outfit for second-place consideration … you remember Legend, right? Ridley Scott fantasy movie. Baby Tom Cruise. Unicorn. Tim Curry as the demon Darkness, chewing scenery more than he normally does, which is to say we're looking at some truly epic scenery chewing here.
You remember that Cruise, as the young adventurer Jack, wore a scale-mail minidress in this movie, right?
"Rebecca, it's not a minidress, it's a tunic. And it's a completely reasonable thing for someone to wear in the context of a traditional fantasy aesthetic!"
Nein, nein, and nein again. It is a scale-male minidress. Where are your pants, Tom? Leggings? Nylons? Something? There's just some nether-brushing fringe protecting your lower half from the elements!
Cruise flashes those gams and covers himself in glitter (there's a lot of glitter in Legend) like he's gunning hard for the "condragulations" on a D&D-themed RuPaul's Drag Race challenge. You, my good man, are trekking through the forest, bushwhacking your way through who knows what sort of fantastical flora and fauna. He's going to get poison ivy in places no poison ivy should ever be.
This outfit, as originally conceived in the brain of costume designer Charles Knode, might even have started out as a crop-top for a more conventionally proportioned actor since, as we all know, Tom Cruise is the size of a LEGO minifig. Tom Cruise short joke, hey-ohhhhh!
I'm going to do what I have to do to bring about a world where the legendary (ha) "Dark Lili" dress Mia Sara wears in Legend takes Tom Cruise's nerd couture out back and gives it the mother of all noogies.
It will involve somehow giving clothing the ability to move (and dunk on each other with extreme prejudice) independent of their wearers, but I read The Secret (no, I didn't read The Secret) and I think I can make it happen.