Obviously, Mario and Sonic themselves are the most popular. They are the main draws as they are the ones 'meeting' in this fanfiction concept.
Princess Peach might be a popular character too. Her role as someone Mario often rescues could be turned on its head in the fanfiction. Maybe she could be the one to help Sonic and Mario in a new and unexpected way.
In addition to the main characters, characters like Knuckles and Toad could play important roles. Knuckles could be the muscle, protecting the group from physical threats. Toad could be the guide, showing Sonic around the Mushroom Kingdom. And of course, Dr. Eggman and Bowser should be there as the main antagonists. Their combined evil plans would be what Sonic and Mario have to overcome.
One interesting concept could be the unique power combinations. For example, Mario's jumping abilities combined with Sonic's speed could lead to some really cool adventure scenarios. Maybe they could team up to defeat a common enemy that neither could handle alone.
Mario and Sonic are at the top of the list for sure. But in addition to Luigi and Tails, Princess Peach is often included. She can bring an element of needing to be rescued or she could also be a strong character helping out. Knuckles from Sonic's world is also popular. He has his strength and knowledge of the Chaos Emeralds, which can be crucial in a crossover story. There are also some fans who like to include Bowser and Dr. Eggman as the main antagonists, which makes for an exciting story.
Sonic himself is usually really popular. Whether it's the Sonic from Sonic X or Sonic Underground, he's the main hero and often at the center of the action. People love reading about his adventures and how he deals with the challenges in the combined universes.
Sonic himself is always a popular character in this type of fanfiction. Whether it's Sonic from Sonic Boom or the modern Sonic, he's the central figure. Tails also tends to be popular as he's Sonic's sidekick in both versions. His intelligence and ability to create gadgets can play important roles in the crossovers.
One idea could be that Sonic and Mario have to team up to save both their worlds from a new, powerful villain who has the ability to manipulate time. This would force them to use their unique speed and jumping abilities in new and creative ways.
Bowser can also be a great addition. He can be the antagonist who tries to cheat in the Olympics or cause trouble for Sonic and Mario. And from Sonic's side, Knuckles is a fan - favorite. His strength and gliding ability can make for some interesting interactions with the Mario characters. For example, he might get into a friendly competition with Donkey Kong over who is stronger.
Mario and Sonic themselves are the most obvious and popular choices. They are the main characters of their respective franchises and their rivalry is always exciting to explore.
Princess Peach from the Mario universe is often included. She can add a touch of elegance and sometimes be a damsel in distress or a helpful ally. Tails from Sonic's world is also popular. His intelligence and flying ability can be useful in various situations in the fanfiction, like helping Sonic and Mario solve puzzles or reach high places during the Olympic events.
Mario and Sonic themselves are obvious choices. They are the main characters of their respective series and their rivalry - turned - friendship can be a great focus. Luigi is also popular as Mario's brother, and Tails is a fan - favorite in the Sonic series. Tails' intelligence and his ability to fly can add an interesting dynamic to the story.