The best class is the one you spend the most time and effort into mastering. For instance, I usually play as Demoman and I'm best at it since I've had the most practice with him. It also depends on what you mean by best. Because if you want a class that will kill lots of people, you are not going to play as medic or spy. If you are just starting out, I'd say Spy is the worst to start off with. He is generally the hardest class to master and you will destroy yourself if you don't have enough patience to keep practicing. I'd say Pyro and Soldier would be good to start with as the control over these two class and pretty simple- just run around and shoot stuff.