1. Iron Man - RDJ bought his A-Game to this movie, and when he does there are few actors in the world that can hang with him. This movie doesn't work if they don't get this character right. It would be all too easy for Iron Man to head down the road to being an outright villain (see the comic version of Civil War), but that's not a problem here. He's the best character in the MCU and this is best he has been used since Iron Man 1.
2. Captain America - Chris Evans is excellent as always. The core of this movie is the relationship between Iron Man and Captain America, and if either one of those characters were a let down the movie would suffer greatly. Fortunately, that is not the case.
3. Spider-Man - Not a major role but a major presence. An enormous crowd-pleaser who ended up being one of the film's highlights in his short screentime. I thought he was great and the best portrayal of Spider-Man in a long, long time.
I almost put Zemo in the third slot, but I just couldn't leave Spider-Man off the list.