RE: Which plane would an instructor fear the most?
I agree with NorforkSouthern on the Raptor. We had a novice pilot show up at the field with a new Raptor and wanted some help getting it set up and training. I went over the plane and run a couple of tanks through it before I took it up. It was a pig. I checked the stall characteristics at altitude and decided that I would have to keep the speed up on final. It took me two passes before I got it on the ground and for the next flight I turned it over to one of the other senior pilots. Even with a few words of warning he stalled it about two feet off the ground on landing and broke the nose off right at the second bulkhead. The new pilot was very disappointed and that was the last we ever saw of him.
There is one plane that I have never flown and have refused the opportunity when offered. That is the GeeBee. I have never seen one complete the first flight. They always look like a bull ride at the rodeo before they end up as a pile of sticks.
As trainers go my least favorite is the NextStar. It is a flyable airplane but goes at the bottom of my list as a trainer. At the top of my trainer list is the SIG LT-40. Anyone that can’t learn to fly that will never be able to fly at all. The Senior and Seniorita round out the top three.