GTO, simply because I think after it's disasterous re-design (note: I'm not talking about power), it deserves another chance.
-AND-
the Camaro's already built (almost),so let the Firebird stay Dead. It was a nice Car, but a rip off on the Camaro; in good way
-AND-
Realistically, if GM is Really trying to achieve brand differenciation, then the Firebird cannot come back anytime soon. (or at least, it can't come back as the Camaro's twin brother...)