I really think Infinity War is the best, because of the film's unpredictability. This is the only time I really felt that there were serious stakes at hand, where anyone could get killed (save the new franchise characters). I didn't get that sense in Civil War, which lessened my interest in major events like the airport fight scene.
Also, Thanos' performance and characterization is a highlight in the MCU.
This is also, for me, the best paced Marvel film. The adrenaline of the action rarely lets up, because everyone is preparing for Thanos' arrival. The character interactions are pretty much all while they are on the move to the next objective.
I felt exhausted after watching this film, similar to the feeling after a roller coaster ride. It's such a fast-paced, unrelenting experience. And no MCU film has given me this feeling until now.
It exceeded my expectations -- big time.