First class I bought with my free 250 gold was the Doombringer class. Definitely worth it, due to the homing missiles you get to use. Many people get pissed off at those, especially flag carriers. Not quite so useful in tight areas, especially if there's a Juggernaut about.
What's even better is that I've fought off numerous assassination attempts whilst camping with the missile launcher due to the class having lots of health. Simply spin around and rev up that chaingun.