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.
It could be Team Rocket. In the Pokemon world, they are always causing trouble. When Sonic shows up, they might think he's some new kind of super - rare Pokemon and try to capture him. Their bumbling attempts to catch Sonic, who is much faster and more elusive than any Pokemon they've dealt with before, would be really hilarious. And Sonic would probably just outrun them easily, leaving them scratching their heads in confusion.
Sonic and Pikachu would be really interesting. Sonic's fast - paced and adventurous nature combined with Pikachu's cute but powerful personality could lead to some great comical and action - packed scenes. For example, Sonic could be running around really fast and Pikachu could be trying to keep up on his little feet while zapping enemies.
Classic Sonic could be the most interesting. He represents the roots of Sonic. When he meets Modern Sonic, he might be curious about all the newfangled things like Sonic's modern shoes or his new power - ups. His reactions to the modern elements would add a lot of charm to the story.
Sonic himself would be very interesting. Since there are two different versions of Sonic in Sonic Underground and Sonic X, it would be cool to see how they interact. They could have a friendly competition about who is faster or better at saving the day. It would also be interesting to see how Sonic from Sonic X reacts to Sonic Underground's family dynamic, as Sonic in Sonic X is more used to his friendship with Tails and Knuckles.
I think the interaction between Sonic X's Knuckles and Sonic Boom's Knuckles would be really interesting. They both have their own sense of duty and pride, but their approaches might be different. Sonic X Knuckles is more traditional in his guarding of the Master Emerald, while Sonic Boom Knuckles might be a bit more laid - back. It would be fun to see them clash and then come to an understanding.
Shadow the Hedgehog and Cerebella would be a fascinating match - up. Shadow's brooding and mysterious nature could be intrigued by Cerebella's confident and somewhat flamboyant fighting style. They might start off as rivals, constantly trying to outdo each other in combat, but gradually come to respect each other. Shadow could learn to be a bit more open from Cerebella, while Cerebella could learn the value of Shadow's more strategic and calculated approach to battles.
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.
Definitely Garfield and Leon. Leon is such a strong and respected trainer in Pokemon Sword and Shield. Garfield, on the other hand, is this lazy cat. The contrast between their personalities would make for an interesting interaction. Leon might be really confused by Garfield's lack of interest in becoming a great trainer and just his focus on food.
Sonic, of course, is a must - have. But also Knuckles from both universes. In Sonic Boom, Knuckles is a bit more of a muscle - brained character in a fun way, while the classic Knuckles is more serious about guarding the Master Emerald. When they meet, there could be some hilarious misunderstandings at first, like Sonic Boom Knuckles trying to show off his strength to the other Knuckles, but then they end up teaming up against a common threat.
Hulk might be an interesting one too. Sonic's speed could either annoy or amuse Hulk. If they end up on the same side, Sonic could use his speed to lead Hulk towards the enemies. And Hulk's raw strength could be something Sonic would be in awe of. They could have a really unique dynamic, like a fast - moving pest annoying a big, strong giant at first, but then they become an effective team.