The bravado answer is that pilots have no fear in an airplane. I strongly disagree. I think that is a dangerous attitude. I have fears, but I channel those in helping me be an extremely careful pilot. That fear makes my preflight checks thorough. That fear reminds me to double check the weather before I takeoff. That fear keeps me from deferring maintenance that might not yet be a problem.
But, having said that, I certainly don’t fly crippled by abject fear. A healthy dose of mortality is useful, but allowing it to be controlling isn’t.
But to answer your question directly, the thing I fear most in an airplane is this: