Difficult, apart from one ofs and prototypes, like the TSR 2, (I've seen both of them but not flying obviously) it would have to be some of the Soviet types. Trapped a Bear, Blinder & Backfire but have never seen a Blackjack. Have caught Fulcrum, Flanker & Fishbed but never Flogger, Fitter, Fencer or anything else beginning with an F. Been lucky enough to meet Coot, Coke, Coal, Colt, Camber, Classic, Crusty, Careless, Cock, Cub & Candid... But never Cossack, Charger, Curl, Cleat or Cline. Hip, Hind & Hook are the only 'H's I've ever seen.
Vaguely aircraft related, I'd love to see an ICBM/SLBM launch (test shot obviously!)