Based on this season, voted for Pogacar and Sivakov. Former doesn't need explanation, but the latter showed he can handle 3 weeks of racing while being a strong TT-er and a solid climber. Crucially, he is in a team, that has in the last decade excelled at developing exactly that kind of GC-riders.
I voted the exact same way, for the same reasons. I think they'll both win a GT in the next 5 years. Sivakov's upside is tremendous; I think he'll be a better climber than Pogacar.
I can think of disqualifying reason for everyone but Remco on that list, but I just have no idea how his career will pan out. It's hard to see a DQS rider winning a GT, however, the way the team is currently constituted. I'm most disappointed in Landa and Lopez, both of whom a few years ago looked like locks to win a GT.
I wouldn't totally rule out Buchmann, but I always get him confused with Ackermann, Schachmann and Schwarzmann. So there's that.
(Kidding a bit -- I think Bora could actually put together a GT winning team, maybe not with Majka as leader, but so long as they have Sagan and Bennett and Ackermann that probably won't happen)
Edit: I thought long and hard about Kruisjwijk, who's podiumed in 2 GTs without great team support, but I think there will always be one or two riders who will outclass him. Of the other young riders, I need to look at Sosa and Higuita a bit more but both are really promising.