Medic, oh so simple, or engineer.
A fully trained medic can move from one side of 2fort to the other without actually having to touch the ground (double conc is my specialty). Infect one ignorant HW and there whole team will rue the day you switched teams.
An engy with a fully complement of EMP's is bacically unstaoppable except from a snipers bullet, even then the emp will leave there hand before they hit the ground. I made a script to automatically throw a bag with the emp to cause max damage (it can even kill a scout in 1 toss).
EDIT: I don't know about that starlifter, as a former member of an all-engineer guild I know how to handle the corners of the flag room. Dispensers are the most effective early warning system.