St. Louis and Louisville really got after each other right from the start. Online anyway... The St. Louligans and The Coopers had some great social media banter.
Charleston, Richmond and Rochester have been going at it for years. I'm pretty sure you have to exist for more than a year before you can claim the title of best rivalry. To be honest, the S2 and T2 battles have so far had the most intense rivalry. And thats just bleed over from MLS etc.