I was best man at a friend's wedding. Can we use "best man" without an article? Thanks.
Yes, this is correct without a or the because there is only one best man at a wedding. You can do that same thing for similar nouns: He was captain of the football team. She was chess champion of the world. I was president of the debating club.
But if there can be more than one with the same title, you would use a or the. I was a groomsman at my friend's wedding. (Not: I was groomsman.) She was a vice-president of the company. See this answer on ELU.
Although to me, we can say He was co-captain of the football team even when there are more than one co-captains.
Yes, colloquially, I hear this type of statement often.
While most commonly you would say "I was the best man at a friend's wedding.", colloquially, I have often heard it said without the article. Spoken speech is often more informal than when it is written.
This omission of articles has also been addressed here; When can an article be omitted?