ok, the all time hardest boss? easy. Sorry if this has already been menioned, but I really can't be bothered going through 140+ pages of replies.
There is a game called Slime Volleyball. If you havent played it, go here:
http://oneslime.net/
The final boss is this evil flashing slime that is faster than you, and always screws you over. Then when you finally beat it......you have to play it again. This time without making a single mistake, as now instead of having to beat you six times, and vice-versa, it only needs to beat you once, but you have to beat it the full six times.
Most. Annoying. Boss. Ever.
