Organize (Rank) the Position Groups by Difficulty

Should we disregard specific schemes and focus on the general cases? There are many factors that may deserve consideration, but that'll complicate things unless we narrow the topic down to a specific style of play.

I'd like to read your opinions regarding the position groups first because the main question/task here is tough.

I hope to find a few arguments that I could stand behind regarding which position groups are the most difficult.

The Position Groups
Quarterback
Offensive Linemen
Running Backs
Receivers/Tight Ends
Defensive Linemen
Linebackers
Defensive Backs
Kickers
Return Men
Blockers (general; kickoff, punt, offensive situations)
Punt Team
Kickoff Team

I'll tentatively state that the secondary is the hardest group to play. I believe defending the run is easier than defending the pass, but I'm not sure how to proceed with arguing that point. In general, offensively I believe quarterback is the toughest position group.

Overall, I think I'd lean towards cornerback then maybe safety being the two most difficult positions to play, but I'm open to opposing views.

I shouldn't have to say this, but you aren't obligated to critique every position group. Please critique the ones you know or have observed well. I'd like to hear your strongest opinions. Let's keep it civil though.