Maybe a new, self - created super - villain. A scientist who has been obsessed with gamma radiation and tries to create his own version of She - Hulk but ends up as a disfigured and powerful monster. He blames She - Hulk for his misfortune and wants to destroy her, and Spider - Man gets in his way.
The Kingpin could be a great antagonist. He's always been a thorn in Spider - Man's side, and his criminal empire and resources would pose a major threat to both Spider - Man and She - Hulk. He could hire super - powered mercenaries to fight them.
The Abomination could be a great main antagonist. He already has a history with the Hulk and his hatred for Hulk could extend to She - Hulk as well. He could be trying to create a new gamma - based weapon to finally defeat the Hulks once and for all.
I'm not sure who the absolute best is, but there are some well - known writers in the fanfiction community. Some writers who have a great understanding of both characters' personalities and powers tend to write really engaging stories. You can often find them by looking at the reviews and ratings on fanfiction websites.
Kingpin could be a great antagonist. He already has a history with Daredevil and his criminal empire could pose a threat to both She - Hulk and Daredevil. He might try to use his influence to turn the public against She - Hulk and create chaos in the city.
Doctor Octopus is another option. His mechanical arms and high intelligence could make for a formidable foe. He might see Deadpool and Spider - Man's partnership as an obstacle to his grand plans. He could try to outwit them and trap them in his elaborate traps using his scientific knowledge.
I'd recommend starting with the most popular ones on websites like Archive of Our Own. They usually have a good summary that can give you an idea if it's the type of story you'll like.
One of the popular pairings is a platonic friendship between Spider - Man and She - Hulk. Their different personalities and abilities make for an interesting dynamic where they can support and learn from each other. Another pairing could be a romantic one, although it's less common. Some fans might enjoy exploring the idea of a relationship between the two, imagining how their powers and personal lives could blend together in a romantic relationship.
The main antagonist might be a being from another dimension who is able to manipulate both magic and the Hulk's gamma - irradiated energy. This being wants to use the chaos caused by the meeting of Harry and the Hulk to open a portal to let in more of his kind to take over all the universes. He sets traps for Harry and the Hulk and constantly tries to keep them from working together to stop him.
The Green Goblin could be a great main antagonist. He is a classic Spiderman villain, and his chaotic nature would clash well with the combined forces of Spiderman and Red She - Hulk. His glider and pumpkin bombs could pose a serious threat to the city and force our heroes to work together in new and interesting ways.
One great Spider - Man and She - Hulk fanfiction is 'The Team - Up'. It has an interesting plot where they have to solve a mystery in New York City together. The way the author writes their interactions is really engaging.
Maybe the Vulture. He's always on the lookout for new tech to sell on the black market. When he hears about Ant Man's unique shrinking abilities, he decides to capture Ant Man and force him to reveal his secrets. Spider Man steps in to protect Ant Man, and they have to fight the Vulture together.