Joseph,
Just as you probably wouldn’t describe her as your roommate, I would think that describing her as your wife is more appropriate than describing her as your friend.
I agree with you 100%, without a doubt. I think the reason “my wife is my best friend” has become a term of endearment is due to the surrounding culture, which presents a toxic view of marriage. Wives called the old ball and chain, or the nag. The ridiculous non-Jewish custom to have one last “fun” party before the wedding, implying that the “fun” stops after marriage. The pervasive message that husbands and wives don’t enjoy each other is terrible, and I personally don’t mind any attempt to counteract that. When someone asks, “who is your best friend?”, they are asking who is liked the most, and I think it’s totally appropriate to put one’s spouse there, even if “friend” is not a comprehensive term to describe the marital relationship.