Tony- BP. It stands for Billionaire, Philanthropist since he is no longer allowed to be a playboy
Bruce- Number 1 since he's your number one and you think he's smarter than Tony
Loki- Sub-Zero because they both have ice powers.
Clint- Clinty-Poo. He actually loves getting called that by you.
Steve- Hun as it's a variation of what he calls you.
Thor- Cutie. He actually hates getting called that but can't bring himself to tell you.
Pietro- Pie. Not that original since it is just the first three letters of his name but you love calling him that.