Young Guns have been the iconic rookie subset of the flagship Upper Deck sets stretching back (with some gaps) to the initial 1990-91 set featuring Fedorov and Bure, so there's a nostalgia element to their appeal. As they're short-printed in modern sets, but are still relatively affordable, if folks are going to get only one RC of a player, they usually go for the YG.
OPC, Parkhurst, and MVP are seen as lower end sets. OPC Platinum Marquee Rookie parallels are pretty popular for people that want to get a rainbow of a particular player, while SP Authentic Future Watch Autos are probably second most popular after YG for people willing to hit a higher price level without emptying their wallets for The Cup rookie patch autos or whatever.
jimetal 7212 and BuccaneersDen mentioned that YG have the full array of rookies spread amongst Series 1, 2, and extended, while OPC ends up excluding some players because of timing of the player's first game. In recent years, Upper Deck has done an OPC Update insert subset in flagship series 2 packs to hit Marquee Rookies for guys like Adam Fox and Jake Oettinger that weren't able to be included in OPC.
Getting out of the hobby, only looking to sell at this point. Also slowly listing on eBay, BSC, and Whatnot if you prefer to buy there for some reason.
$15 credit available for new users with my Whatnot invite.
Cheers!