It was Cloud for me in Smash 4. While I don’t necessarily care for Cloud himself, but him joining just really felt like anything was possible. So most characters afterwards were pretty obvious ones like Steve and Incineroar or ones that were underwhelming but still fine like Min-Min and Pyra. Although Pyra had a little twist on it.
Oh actually no I LIED.
Piranha Plant wins for me. Even when Piranha Plant was revealed I was confused. I never expected it. The surprise for it is what makes me love the reveal.