That's not really a leap of faith, since you don't make the landing.
I think the developers may have said at the time of Brotherhood's release that the Castel Sant'Angelo was the tallest building in the series. That's only because of the huge flagpole they put on top of it, though.
As for the tallest leap of faith, it may well be the Cathedral in Acre in AC1 that Bobby mentioned. Giotto's Campanile in Florence in AC2 is another possibility (there was an achievement/trophy specifically for Eagle Diving from the top, and although the summit of the Duomo is higher it takes two dives to reach street level), or the tower besides the Basilica di San Marco in Venice in AC2.
''Snake from Metal Gear 2 owns everybody. Because he used an owl. To make a guard think it was night time. In the middle of the day. That's just hardcore''