By position:
QB Highest ceiling Matt Stafford or Andrew Luck, probably seen 40+ NFL QBs in HS. Kyler Murray was better than both in HS, though, and not close if HS is the criteria.
RB Cedric Benson
WR Roy Williams or Dez
OL Eric Winston mauled people at tight end, I'll go with him
DL Nikita Whitlock, Von Miller, or Ashawn Robinson.... all 3 were insanely good in different ways
LB Derrick Johnson (I saw Armstead but I was really young)
DB Probably Kenny Vacarro at safety. I saw him destroy a dude Sean Taylor style without pads in 7on7 and I thought he straight up murdered him. He was promptly ejected... I did not think he would make it to the NFL with his attitude but he's done well. A defensive version of Aaron Hernandez in a lot of ways, miracle he's held it together.
KR/PR Quan Cosby
Best all around high school player I ever saw might have been Rex Burkhead. He was the best running back in the state and the best safety in the state his senior year. I've never seen a kid dominate in all phases quite like he did. If I had to pick one person on this list for my HS team it would be Murray or Burkhead despite neither of them being high ceiling NFL guys.
It is a shame that Cedric Benson (a) was a total knucklehead and (b) went to the University of Texas. He never developed into anything close to what he should have been after high school. Quan Cosby, ditto.... tore up his knee, played minor league baseball, and was never what he could have been. He looked like an alien playing small school football.
I've seen some really cool matchups. I saw Cedric Benson play Derrick Johnson.... Benson's high school's song used to be Dixie, and their band paraded around the stadium multiple times in front of Derrick Johnon's team's bleachers (Waco High). The black folks from Waco (i.e. 99% of the fans) were not amused to say the least. Amazing game Benson barely escaped with and Johnson won almost single-handedly.