It could be Doctor Octopus. He's always been a formidable foe for Spider - Man. In this fanfiction, he might have a plan that involves not just Spider - Man but also threatens the Avengers in some way, so they have to fight him to protect Spider - Man.
The Vulture would be a great choice. His flight abilities and scavenger - like nature could pose a unique threat to the Avengers. Spider - Man already has a history with him, and it would be interesting to see how the Avengers deal with a villain who is more used to operating on a smaller, street - level scale like the Vulture.
It could be Mysterio. He's really good at creating illusions and making Spider - Man look bad. In this case, he might be tricking the Avengers into thinking Spider - Man has done something wrong, which forces Spider - Man to hide from them.
It's really hard to say. Spider - Man is extremely agile and has his spider - sense which gives him an edge in combat. But the Avengers have so many powerful members. Iron Man has his high - tech suits, Thor has his hammer and god - like powers, and Hulk has his brute strength. If it's a one - on - one fight against most of the Avengers, Spider - Man might be at a disadvantage, but if he can use his environment and his web - slinging to his advantage, he could hold his own for a while.
Well, it could start with Spider - Man being framed for a crime he didn't commit. The Avengers believe in his innocence. They fight for him against a corrupt government agency that wants to arrest him. Along the way, they discover that there's a bigger conspiracy at play. Black Widow uses her espionage skills to gather intel, Hulk provides the brute force when they face the agency's armed forces, and the other Avengers contribute in their own ways to clear Spider - Man's name and protect him from harm.
It could be Loki. He's always causing trouble for the Avengers. He might see kidnapping a fan as a way to mess with Spider - Man and the Avengers, since he enjoys chaos and manipulation.
Perhaps Sabretooth could be the villain. He might be targeting Wolverine specifically, but his actions in a big city draw Spider - Man's attention. Sabretooth's savage nature and powerful claws would pose a great threat. Spider - Man and Wolverine would have to work together to defeat him, with Wolverine having the personal vendetta and Spider - Man the sense of justice for the city.
Spider - Man would probably be shocked at first. He looks up to the Avengers and being kidnapped by them would be the last thing he expects. He'd likely try to talk his way out, using his quick wit and charm. He might start by asking why they're doing this and try to convince them that they've made a mistake.
Spider - Man would be very confused. He would feel betrayed because he thought the Avengers were heroes he could trust. He could start by asking them why they are doing this. If they don't give him a satisfactory answer, he might try to escape in a sneaky way, using his spider - like abilities to find a weakness in their capture setup while still trying to reason with them.
Spider - Man would probably be defensive at first. He's used to working alone most of the time, and being interrogated by the Avengers would seem unfair to him. He might crack some jokes to ease the tension, but also firmly state that he's a hero too.
In some fanfictions, the winner isn't really about who is more powerful physically or technologically. It could be about who has the stronger will or the better moral compass. For example, if Iron Man realizes that he's in the wrong during the fight, he might back down, making Spider - Man the 'winner' in a sense. Or if Spider - Man is able to convince Iron Man of a greater good, the fight could end without a clear physical victor.