I imagine that most of the man-made acoustics with the longest reverberation times will be Cathedrals.
Here in the UK, Liverpool's Anglican Cathedral has a reverb time of around 11s (I think, though I can't verify immediately), and I suspect that St. Paul's in London is at least as long if not longer, as will other similar buildings.
I wonder what Barcelona's Sagrada Familia is (or rather will/would be once complete)?!
AVI