Tony: You call him 'My Inventor,' 'One and Only', and his personal favorite: 'Billion Dollar Baby'
Steve: He is, was, and always will be Dorito, Stevie Wonder, Captain Sexy, Ice Cube, and Dear.
James (Bucky): He really loves it when you call him Sarge, but he loves it even more when you walk up to him from behind and mumble a sleepy "Hi James" in his ear as you hug him. My Hero is another one that never fails to make him smile.
Quill: Spaceman. When you call him Star-Lord he can't help but kiss you.
Natasha: You just call her Nat, Tasha, and Красивый(pronounced Krasivyy) (Beautiful)
Sam: You call him a lot of names, mostly insults. (*cough*birdbrain*cough*) Most often though, you call him your nest because one, bird puns for the win, and two, he really is where you feel most at home.
Loki: My king. God, you love the smile he gives you when you call him that.
Peter Parker: Spidey, Pete, Peanut Butter, (look at his initials. PBP. PB) and handsome. His favorite is Precious One though.
Clint: One word- Schatz. It means treasure in German.
Wanda: My love. When you're sleepy, she's Scarlet. Both she finds cute.
Pietro: No one word did him any justice, so at first you called him Pietro. As time went on, you changed his nickname to Серебряный король(pronounced Serebryanyy korol') (Silver King)
Peggy: Darling, Dearest, My Love. She doesn't need anything fancy, she says. You love to call her Pegs.
Wade: Baby, Precious, Headless Horseman (he once showed up at your door with his head barely connected. It was Black Friday, and he was trying to give you a stuffed horse) You called him My Jewel a lot too. (I once had a gf I called My Jewel. She was cool.)
Nick: Fury doesn't really like pet names, but he can't hide his smile when you kiss his cheek and call him Nicky.
Stephen Strange: Master of Time, Love, and his personal favorite, Doc.
Scott: You call him Tiny a lot, but at the end of the day, it's Sweetheart you say goodnight to. Oh, and your hero.