Gotta break it down by category...
WWII: P-38, Mosquito, & the FW190D
Still trucking: B-52, C-130 Both look set to outlast multiple replacements
Mud-Movers: A-10 Warthog, the only plane the US Air Force can't kill no matter how hard they try...
Mach 2+ SAM Sites (aka: fighters): F-22. I got to see one running through its paces at the last NAS New Orleans air show. Bloody thing manuvers like a goddamn UFO. Seriously. I watched this thing do a bootlegger turn. Not an Immellman, not a Split-S, but a flat 180 degree skid and acceleration back the way it came. At a fairly decent subsonic speed. What it did at low aerobatic speeds defy description. Iranian & Russian fighter pilots should just save themselves a bunch of trouble & hit the silk as soon as they hear that hostilities have commenced.