The worst flying planes I've ever seen are the WWI biplanes and Triplanes.

They seem to have more drag than lift when the engine is off
The tail gets blanked out when the nose is up during landing
There's just a tail skid so no steering until you can get the tail up

They are awful in a cross wind

But my fear that has shown up more often than not is the newbe with the P51 showing up at the field. That has to be the most beautiful plane ever made but it is not generally as easy to fly as its looks are good.