There's not a particular question that's most difficult. I'd recommend being pretty up on RTOS and recent embedded microcontrollers. In my case I feel I am rather up on C, C++, board design and bringup, hw tools, author-level linker and debugger tech, fair RTOS understanding at a kernel level, register-level ARM and AVR understanding, and decades of UNIX experience, and those didn't get me to "yes", despite all those fitting the job description. So you might wish to set expectations that a whole constellation of judgement may be at play and a good technical show may be just half of the requirements. I really don't know why I wasn't selected, unfortunately, so my guess is in a market like SF or Seattle one gets a pretty sweet pool from which to choose.
