It matters what game you are playing.
For example i play Wizard101 and so far the hardest boss to me is Jabberwock.
Search 'the hardest video game boss ever' on YouTube to see who she is.
Ozma is the hardest boss in the game
orcram
The hardest boss is Sigma
you
the dog
The Moon Lord
The next hardest boss I'd say is Kolokotos.
Megahammer in world 3 ( Finel Boss in world 3)
the hardest boss in kingdom hearts recoded would either be roxas or sora's heartless in it's last form
the hardest pitch ever was 106 mph
Nemesis