Without a doubt the manual selector switch. The trigger is just a big no-go for me. I don't want to have to think about how I'm pulling the trigger to determine whether I am getting a single shot or string of shots, I want to just be able to pull the trigger the same way for either ad get what I want. The trigger system is not a good design for shooting with adrenaline going as it has a much higher learning curve.
My only experience with the trigger type selector is from an F88, I'm not aware of other weapon systems that use this.