In my opinion, a great crossover fanfic is 'Harry Potter and the Star Wars Universe'. This fanfic takes the magic of Hogwarts and mixes it with the epic space battles of Star Wars. The concept of Hogwarts students learning the ways of the Force is really interesting. It also has some great character pairings like Hermione and Rey, who could share their knowledge and skills.
One of the best crossover fanfics could be 'The Avengers meet Sherlock Holmes'. It combines the action - packed world of superheroes with the brilliant deductive reasoning of Sherlock. The author manages to blend the two universes seamlessly, creating exciting new storylines and character interactions. For example, Iron Man and Sherlock might engage in a battle of wits over a mysterious device.