A best bud is a closest friend, someone you trust and rely on more than anyone else. It's casual slang used to describe a tight bond, shared laughs, and steady support. People often use it for a long-time friend who feels like family.
I got my service dog when I was medically retired out of the military, and it was the best thing that ever happened to me. I wish every medically retired serviceman could have a service dog. He's amazing. He's my best bud. I go everywhere and anywhere with him. Marcus Luttrell