It could be the newer protagonists who are more surprised. For instance, if a protagonist from a more recent game meets Ash, who is like a legend in the Pokemon world. They would be in awe of his adventures and his reputation, while Ash might be more used to strange encounters compared to them.
All of them could be equally surprised in different ways. Each protagonist has their own worldviews shaped by their respective adventures. So when they meet, they are all likely to be taken aback by the differences and similarities in their experiences, Pokemon teams, and training methods.