I do not know the most offensive art project extant, although I'm sure Piss Christ is somewhere in the top 75.
Friend DEG suggested one to me a while ago.
He said, "Did I ever tell you about my idea for the most offensive art installation in history?"
"It's called 'Jesus fucking Christ.'"
"That's a verb, isn't it."
So there you have it. An image to keep you going, brought to you by the guy who gave local band The Inflatable Jesus Love Dolls the name for their song 'Big Penis Truck.'
But the question is: should we be allowed to suggest it? In a day and time when people worry about a political correctness that never was there is a strong, strong fence around criticism of religion and patriotism. We can't probe belief, investigate it nor ridicule it. Anything can be defended by the words "Well, that's what I believe." The very people who are against any form of relativism are all for sliding behind a brick wall protection of belief. They wouldn't want to see light shone into dark sections of there world view.
So my answer is yes, like burning a flag or swear words or any number of things I don't like and don't want to hear, it's not only allowed but, from time to time, required to be absolutely offensive.