Jacked up and good to go
They may not have the numbers of the Zerg, or the technology of the Protoss, but who needs that when you have steroid-crazed Marines mowing down anything that moves? Let the other guys send in their fancy robots and weak underlings - they'll all fall under a hail of bullets and concussive grenades. And if it looks like you're about to be overwhelmed, just drop a barrage of nukes.
That's right. Nukes. How could the Terrans NOT be the best race?