Bruce- Steve. Your friendship started when you taught him how to use his cellphone.

Tony- Bruce. Since you work together it seemed only natural that you would become best friends.

Clint- Tony. He's your drinking buddy.

Thor- Loki. Although you claim him as your best friend he denies it.

Loki- Clint. You always felt bad about what Loki did to him so you were always extra nice to him. That somehow merged into mutal friendship.

Steve- Thor. He's actually more like a personal bodygaurd/shopping bag holder but with the title friend.

Pietro- Clint. The two of you are best buds. Although when Pietro came your relationship with Clint was strained but after Pietro saved Clint the hawkwardness left.