It depends on what it is. If it really is Coca Cola, I'll say coke, b/c Coca Cola and Pepsi are brand names, and taste different (or so I've heard ) The other English teacher last year said pop, and I've heard other people call it soda pop, but I say soda. I don't like soda, so I don't say it that much!!
I guess it's kind of a north-east thing, like dog is kind of like dawg, and coffee is more like cawfee, and talk and walk are like tawlk and waalk. that didn't look right!