Comparing Scully to Petracca as a 18 year olds, the only thing i'd give to Scully is cleaner hands in close, better handballer, and endurance.
Petracca is clearly the better athlete. Is Bigger, stronger, more aggressive. Is a much better mark, kick, and tackler. He is able to hurt teams offensively on the inside as well as the outside and kick goals much better then Scully was.
As a 18 year old due to his good hands in close and the smaller bodies he was playing against, Scully was probably as dominant on the inside as Petracca, but due to his small size it's not a dominant skill as transferable as Petracca's . Petracca's a likable kid and team mates seem to like him, unlike Scully
Winner Petracca
I'd Rate coming out Trengove and Brayshaw pretty similar, Both as a 18 year old are pretty similar athletically,good speed on first few steps, average top speed though Brayshaw would be quicker, though Trengove would have him on endurance.
Brayshaw is clearly better inside player, with better strength and size, Though Trengove would be the better outside player and is a better kick on his prefered foot, brayshaw would have him on his non prefered. Both showed the ability to kick a goal, Trengove would of been the better mark and better forward. Both showed aggression and willing to tackle, though due to size and strength, Brayshaw would have Trengove on that. Both really good leaders and natural Footballers. As a 18 year old i don't think there was much between the 2 and Can't split them.
TIE
Going on the prospects they were when drafted i'd have to give it to Brayshaw/Petracca over Scully/Trengove, 1 1/2 points to 1