Harry:

What you call him - Harold (kind of a jokey one), Haz, babe, The Chosen One (another jokey one)

What he calls you - Beautiful, love, darling, princess

Ron:

What you call him - Ronnikins (you started calling him this to annoy him but he's actually grown to love it), baby, gingersnap (to annoy him), handsome, goofy/goofball

What he calls you - Gorgeous, cutie, love, baby girl, lovely, a shortened version of your actual name

Hermione:

What you call her - 'Mione, angel, beautiful, sweetie

What she calls you - darling, my love, dear, sweetheart

Ginny:

What you call her - babe, honey, Gin, darling

What she calls you - sweets, love, gorgeous, sweetie pie

Draco:

What you call him: Blondie, babe, baby boy, Prince Charming, Malfoy (when you're mad at him)

What he calls you - baby girl, babe, beautiful, gorgeous, princess