I have many Sunni friends who are best friends of mine but some Shias ridicule me for being best friends with them just because they are Sunnis. They aren't wahhabi salafis just normal sufi type sunnis but even then I'm constantly insulted by our own shia. Am I really doing something wrong by being best friends with Sunnis? Aren't Sunnis also Muslims?

No you are not doing anything wrong. Just make sure they don’t negatively influence your beliefs, and that you are always firm on the path of Ahlulbayt (a).