Quote Originally Posted by Braindead_Paul
Yesterday I changed my mind. My new hardest bosses are Ahriman and Dark Cloud (FF3). I was just too stupid to get prepared properly and couldn't go back to any shops etc.
Exactly the two who I was going to mention, except that...
Ahriman is an optional boss!!!!!!!!!

Other than Dark Cloud, or a great deal of other the near impossibles from FFIII, the answer probably lies somewhere in the more wicked dungeons of FFIV. (I had a miserable time with that game)