Finding the "best" Arabic course really depends on what you're looking for. A few places in Cairo get mentioned a lot, like the American University in Cairo (AUC) and the International Language Institute (ILI). AUC is known for its structured programs, which can be useful if you're serious about academic Arabic. ILI tends to be a bit more flexible, offering a variety of courses, including ones focused on conversational Arabic. Both offer different levels. If you are looking to learn in a group setting, it is important to check if they offer Arabic Group Classes as this can change depending on the time of year.
When you're choosing, think about your learning style and goals. Do you want to focus on Modern Standard Arabic, or Egyptian Colloquial? Do you prefer small classes or larger ones? It's a good idea to visit the institutes, talk to the staff, and maybe even sit in on a class before you decide. That way, you can get a feel for the atmosphere and see if it's a good fit for you.