I really, really love the new character, Black Panther, as well as the performance. The suit is amazing and so is the fighting style. He easily is the most interesting character in the movie. I do wish that his background was given more attention though (how did he learn to fight so well, and how did he manage to own a suit that amazing).
Runner-up is Winter Soldier, who was also the most interesting character of the previous movie in the series, Captain America: Winter Soldier.
The 3-person fight near the end was something special, though its motivation was not very believable. It was rather dark for the movie, and steers too far from what we've come to learn about the characters.
Like so many big action movies, the movies suffers from over-complexity of the plot and action sequences, making it harder to follow and appreciate. It doesn't help that it had a ridiculous number of characters, including 2 surprise additions. Having colorful characters can be exciting, but they simply were not needed for the story.