Okay, besides the usual selection of the Arrow, TSR-2, P.1154, VAK 191 and VJ 101, some oddballs:
The Convair R3Y- which would have been Amazing with better engines, especially as a vehicle for Supply Ops and ASW/MPA work, basically as a Seaplane/Amphibian version of the Hercules and Orion for the Navy, but better.
The Martin P6M- If it had been intended to serve as an Anti-Shipping Weapon, carrying what amounts to a Modified Terrier, used as an AShM, Preferably configured as an
ARM or with
SARH guidance. The W45 Warhead might be a pragmatic solution as would be the improved engine (increasing range to 40nm) from the RIM-2F. Of course, it'd be fully capable of strategic bombing as well, to get the Taxpayer's money's worth.
The YA-10B - Intended as an Adverse conditions variant of the A-10A, would have been better as a Forward Air Controller then the A-10 and might've made a good Trainer as well.
The Convair F-106 C/D - which was to be fitted with the AN/ASG-18 radar and fire control system from the canceled XF-108, and used as an Interceptor, like the Arrow, and was supposed to carry one AIM-47 Falcon, and two AIM-26A Super Falcon/Nuclear Falcon missiles in it's weapons bay.
The Boeing YC-14 - competed with the YC-15 (later developed into the C-17) in the AMST project as a medium range STOL transport to replace/compliment the C-130. With a short takeoff it could carry 27,000lbs, on half the airfield the C-130 needed for such a load, and with a long run, could carry 81,000lbs. All in a cargo bay twice as wide as the one on a C-130. Of course, since, with a range of 3,000 miles, it wasn't long legged enough for the Air Force Chief of Staff, meaning it got shit-canned like the YC-15, (which was developed then into the longer legged, larger, and more limited C-17, which lacks the fucking STOL capabilities it was initially designed for.) The C-14 was probably the better of the two designs with the Engine placement reducing the risk of dammage from unprepared airstrips.
Additionally, we can't forget the Bell Boeing Quad Tiltrotor design concept, and the retardedly awesome concept that is Seaplane fighters, like the Convair F2Y/F-7, and the Saro SR.A/1.