Airframe
Blue,
Gosh, I figured if there was anyone who knew it would be you!
Seriously, you've been at this a long time. What do you think?
Personally, I believe it is the airframe with the lowest total drag. In the case of a delta you have a lot of induced drag coupled with a tremendous amount of "wetted" area or total surface in contact with air.
There are other factors. If turning the airplane is necessary, as in pylon racing, a conventional airframe probably turns better and bleeds off less airspeed. I am just guessing the drag on a delta goes way up in the turns.
For absolute top speed in a straight line? Geez, I don't know. I think it will take some kind of extreme speed competition to bring out the optimum airframe.