The reveal of Gear Second in One Piece is a standout for me. We had zero idea of what Luffy was going to pull off and it was absolutely epic.
Also Regular Show has some epic and silly Power ups sprinkled all throughout it. But the basketball god scene stands out for me as insane and epic.
Another one I love is Kurapika from Hunter x Hunter with his epic Emperor Time ability
And...of course...Super Saiyan. Come on... I can't not bring it up.