Tony -

Long and straight so he can run his fingers through it

Steve -

Plaited pigtails as he thinks it makes you look adorable

Clint -

A high ponytail to keep it out of your way while you're both training

Bruce -

Low pigtails because he thinks they make you look peaceful

Thor -

He loves it wavy with a flower crown in it until you can have a proper crown

Loki -

He loves you to walk round the castle with sex hair