At.
Or the best man at/in our wedding ceremony.
Or the best man on our wedding day.
"I want him to be the best man on our wedding." Why is it "on" or "at" the wedding?
At.
Or the best man at/in our wedding ceremony.
Or the best man on our wedding day.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.