Controversial is defined as:
- large number of upvotes and downvotes
- large number of comments
If you can think of extra criteria, feel free to edit the question.
This answer has +156/-169 with 43 comments. I think it's an especially good candidate for "most controversial" since up- and down-votes are rather balanced.
Depends on how you define it. By pure numbers, I'd say it would have to be this one (now 'deleted') -- 1172 up, 107 down, and 105 comments.
But nobody was arguing about it.