I leave “difficulty” undefined - could be physical exertion, mental judgment, etc.
Some say free safety, some say center or nose tackle.
I would vote for cornerback. Run your coverage every single down, back to huddle, run some more. Then if you make one error in judgment, slip, or just aren’t as fast as the guy you’re covering you shoulder the blame for a score.
Perhaps not the most difficult non-QB position, but I think a compelling argument could be made for place kicker. They are on the field for very few plays, many of them scoring plays, potentially game-winning plays, so the pressure can be extremely high. The rest of the limited snaps they see are regarded as “routine,” e.g. kickoffs and extra points, but if you fuck up more than just a few times (possibly even just once), you’re fired and replaced.
A lot depends on how you define “difficult.”
Certainly, to be a top cornerback in the NFL requires a level of speed that’s extremely rare.
I find it interesting that, if you take the Wunderlic test seriously, the smartest guys in the NFL tend to be the offensive linemen. It seems that there’s more to the job than just brute strength and blocking. Centers, guards and tackles need to be powerful AND cerebral, to know the offensive scheme inside and out, and to be ready to make complicated adjustments on the spur of the moment.
Mentally? Middle linebacker. Usually considered the quarterback for the defense with the responsibility of reading the offense at the line and adjusting as necessary. You also have to be a pretty damn impressive physical specimen. It’s a tough spot.
Physically or “pure” athletic skill set? Cornerback. For the reasons mentioned above.
I don’t take the Wunderlic test seriously.
But even if you do, the fact that a given position scores highest in the test does not mean that the position requires the highest intelligence. It is entirely possible that there is some factor that results in smarter people just happening to be the type of people who end up as offensive linemen. For instance (and I have no data at hand to support this) it may be that the so-called “skill positions” like quarterback, receiver, and running backs are likelier to be targeted for college scholarships and to have heaven and earth moved to get them eligible to play even when they’re as dumb as rocks, while offensive linemen are less likely to get that kind of treatment (perhaps not as true now, but it might once have been.) Or there could be some other factor that causes the phenomenon.
Probably cornerback or left guard, because both are positions that require split-second reactions and exact a large price for failure, one being a big yardage gain or touchdown and the other a blindside hit on the quarterback.
Airman: If you meant left TACKLE, it’s what I would’ve said.