I don't think the response with the most upvotes should be chosen by default, because not all answers are voted on equally. Specifically, one user might not submit an answer until several hours after the question was originally posted. Sometimes these answers are very insightful (perhaps previous comments and answers have made the conversation more focused, or maybe the answerer spent a lot of time doing research). Oftentimes, though, these answers don't garner as many upvotes (partly because people have "moved on" to other questions, and these answers get fewer views).
Here's another case when it might be good to select an answer with fewer upvotes: when a user posts an answer that includes a very funny line, and that humor helps it get a lot of upvotes. One recent answer that comes to mind is the "It's time to eat Mat" answer, which had 36 upvotes when I posted this. (In that example, there were no competing answers, so there were no other answers to choose. Moreover, I'm not saying those upvotes are undeserved – Luke's answer isn't merely funny, it's informative, too. I'm only pointing out that it's hardly inconceivable for an "equally good" answer to have fewer upvotes. As a voter, I might be more inclined to upvote the funnier answer; as an O.P., I might look at the answers in a different light, where the chuckle isn't so influential.)