A-10 Warthog.
Trust me, those final Xen levels were ass.
You missed a jumping puzzle, wandering through the bowels of an alien factory of bland and uninspired design, playing whack the pinata with a giant spider's unitesticle and aiming rockets into the head of a giant uberfetus while trying to avoid being teleported away to one tedious battle after another, only to be rewarded with a brief chat by the gman who makes an offer of recruitment - no cool ending cutscene - nada.
To the person at Valve responsible for this abortion of an ending to an otherwise utterly brilliant FPS Googling for this and happening upon my post here, I'd like to say: "I hope you end up at a new job deep frying chicken you fucking idiot." - Anonymous user on the final levels of Half-Life single player.