Shadow could be a very challenging opponent. He has similar speed and abilities as Sonic, and in a death battle scenario, his more brooding and sometimes ruthless nature could give him an edge. Plus, if the Sonic watch is involved, Shadow might be more willing to use underhanded tactics to get it for himself or to stop Sonic from using it.
Metal Sonic would be a tough foe. He is designed to be an exact copy of Sonic in terms of speed and some abilities. In a death battle situation, Metal Sonic could be upgraded with features specifically to counter the Sonic watch. He doesn't have the emotions that might hold Sonic back, and his sole purpose is to defeat Sonic. He could analyze the watch's power and quickly adapt his own attacks and defenses accordingly.
Dr. Eggman could also pose a huge challenge. He is always coming up with new and powerful machines. In a death battle within the fanfiction, he could create a device that counteracts the power of the Sonic watch. He has the intelligence to analyze Sonic's every move and come up with strategies to defeat him. Also, he might use the chaos emeralds in a more devious way than ever before, making it extremely difficult for Sonic.
Madara Uchiha, if he were to somehow be involved in the Battle City scenario. His power level is extremely high, and his experience in combat is far greater than Naruto's at the start. Naruto would have to really dig deep and use all his skills, plus any new ones he's developed, to stand a chance against Madara in Battle City.
Obviously, Sonic himself is extremely popular. He's the main character, after all. Tails also often makes an appearance as Sonic's sidekick. Knuckles is another popular one, with his strength and connection to the Chaos Emeralds. And then there are the villains like Dr. Eggman who are frequently part of the death battle scenarios in these fanfictions.
There might be a wise old grandfather in Sonic's family. He could be the one who knows all the family secrets and ancient techniques. And there could also be some younger siblings, like a little brother or sister who looks up to Sonic and wants to be just like him. They could all have different personalities, for example, the older sister could be really strict but also very protective, while the younger siblings could be full of energy and curiosity.
One idea could be Sonic facing off against a powerful new villain in a time - based battle. The watch could play a role in controlling time during the fight. Another plot could involve Sonic teaming up with other characters from his universe because of a threat revealed through the Death Battle fanfiction concept. Maybe the watch has some special powers that are misused by an enemy, leading to a chaotic situation that Sonic has to fix.
Well, Dark Sonic might have an edge in terms of raw speed and combat skills. However, Sonic.exe has that element of the unknown and a sort of supernatural horror factor. If Dark Sonic can use his speed to quickly attack Sonic.exe before Sonic.exe can unleash its more insidious powers, then Dark Sonic could win. But if Sonic.exe manages to get its powers off first, like its mind - control or reality - warping abilities, it could easily overpower Dark Sonic.
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.
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.
Classic Sonic and Modern Sonic are the stars, of course. However, Knuckles can also be a very popular character in these fanfictions. His role as the guardian of the Master Emerald in the classic world and his continued importance in the modern Sonic universe make him an interesting addition. He can be the one who initially doubts the partnership between the two Sonics but then comes around. And Amy Rose is another character that often appears. Her feelings for Sonic, both Classic and Modern, can add a touch of romance or at least some emotional depth to the story.
Tails is very likely to help. He's Sonic's best friend and is super smart when it comes to technology. He could build a device to speed up Sonic's recovery or find a way to treat Sonic's injury. Amy is also a possibility. She's very caring and would do anything to make Sonic feel better. And Knuckles might help too. He's strong and could protect Sonic while he's recovering.
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.