Not the Hardest boss but one of my most 'pain in the ass miniboss'. The Tyrant on the plane in Code Veronica, damn bastard knocking me out the back.
Georgia (I think thats his name. The zombie boss in FF8) whilst playing a no Junction game. I gave up on this because of him, oh and Sephiroth at the end of FF7 in a no Materia battle, thats pretty hard.
Sturm on Advance Wars on hard. And Advance wars 2 on Normal. The Last mission on AW2 on hard I found easier than on Normal
And it just came to me. Penance on FF10. aaahhh, took me near 2 hours to beat that bastard. Well more like 8 hours if you include all the times he killed me and I had to start over.
Just completed RE4 on pro. 4th play through. Love that game.