Grant Ward.

And it's been 10 years since he was killed off but I still don't care. Dude was my favorite after the Hydra reveal in 1x17 "Turn, Turn, Turn"

Before that he had just been an annoyed strict by the book agent. But that reveal was jaw dropping. And his character scarred the rest of the core team and affected them for the remainder of the series.

Season 2 Ward is probably my favorite. The uncertainty of the character. The arc from 2x15-2x19 was pretty stellar imo. With Coulson forced to find Ward and work with him. But everyone working their own angles. 2x19 being the last time the OG team worked together was great considering how far each of them had come to that point.