The fade route to the corner of the endzone, because 9 times out of 10, the receiver is 6'5'' going against a 5'11'' corner, who has to play with his back to the ball. If properly thrown, this play either ends in a TD or a pass interference call at a 92.3% rate.
The post corner is always tough. Always good to be followed up by a post in. Finish 'em off with a post corner post - a.k.a. the bolt. Just gotta have a little protection for the QB to allow these to work.
The hardest route to defend is a double crossing route in the end zone...if executed properly it works perfectly as a legal pick play. If you can essentially block the defender while the ball is being thrown to his guy, he is at a huge disadvantage
If the comeback was the hardest to defend cornerbacks would cheat to take that away...there is a difference in the offense taking what the defense gives it versus the hardest to defend. If the defender knows what's coming, the comeback is pretty easy to defend. To me, the hardest pass to defend is one that even if you know its coming there isn't a whole lot you can do about it. A pick play in the endzone is near impossible to defend because its a two-on-one scenerio.