Nautilus on 19 July 2016
There are many, but one of the top picks for me would be Dark Bowser.Not only is a epic fight, but also has one of the best final boss music of all time.
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1
