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.
One possibility is that they would compare their Pokedex collections. Each protagonist has had unique adventures and caught different Pokemon, so they could show off their rarest finds and share stories about how they caught them.
Probably the younger or past versions of the members. They would be seeing all the things they have achieved and how they have changed. It would be like looking into a crystal ball and seeing a very successful future version of themselves.
Sakura might be the most surprised. She has always been very close to Naruto, and seeing him as a powerful future version would be quite a shock. She might be amazed at how he has developed his skills and also how he has changed as a person.
Honestly, it's hard to say. Dumbledore would be amazed at Harry's future self, but Harry would also be taken aback by seeing Dumbledore in a different time. It could depend on the circumstances of their meeting. If Harry comes back to a time when he thought Dumbledore was dead, then he would be extremely shocked. On the other hand, if Dumbledore has been expecting some sort of time - traveling encounter, he might be less surprised than Harry.
Definitely Pikachu from Pokemon. It's so cute and full of energy. In the MLP world, it could befriend Fluttershy because she's good with small and cute animals. Another interesting character would be Charizard. Its big and powerful presence would be quite a contrast to the ponies. And from MLP, Rainbow Dash would be a great character to include as she's adventurous and could have some really fun interactions with the Pokemon.
I think Sonic himself would be the most interesting. He's so different from the Pokemon world. His super speed and his attitude would really stand out. He doesn't have the typical powers of a Pokemon, like shooting fire or electricity. His way of dealing with problems, which is mostly running really fast and spinning into things, would be really unique in the Pokemon world.
Pikachu is always a popular character. It's so iconic in the anime and also has a significant presence in the manga. Ash Ketchum from the anime is also very popular as he is the main protagonist. In the manga, characters like Red can be popular in these fanfics too, as they bring their unique storylines and strong Pokemon - training skills.
I think Gatomon would be a great character to feature. As a Digimon, she has a strong and independent personality. If she were to meet a Pokemon like Eevee, there could be a lot of potential for character development. Eevee's ability to evolve into different forms could intrigue Gatomon, and they could learn from each other's adaptability and survival skills in their respective worlds.
I think Pikachu would be really interesting. It's so cute and energetic. In the fanfiction, it could use its electric powers to light up spooky places that Scooby Doo and the gang are exploring. Also, its reactions to all the strange things in Scooby Doo's world would be hilarious.
Hulk could be the most surprised. He doesn't always pay attention to the subtleties of character growth. But seeing Peter, who was once a small and somewhat inexperienced hero, now be this confident and powerful figure from the future would really catch him off - guard. He might even be a bit intimidated at first by the new - found strength and presence of future Peter.