I can't remember... xD
Instead of telling you the hardest boss I've ever faced I'll just tell you the hardest bosses I've fought in the last months =p
Black Heart Noire - Hyperdimension Neptunia Re;Birth 1
Killachine (first battle) - Hyperdimension Neptunia Re;Birth 1
These battles were insane (and unexpected) difficulty spikes but it was so rewarding when I defeated each of them.