DeSmith and Hill are the ones who have played in the NHL so they'd be the ones with the most short term upside by default. Gustavsson is probably pretty close to the NHL but with the current state of the Senators I wouldn't want to touch a young goalie from that team till they turn things around, which will take a few years.
Long term, there are two who jump out here and those are Annunen and Kochetkov. I see both as their teams' future starting goalies if all goes right.
Husso (he's the back up goalie in St Louis next season as Allen won't be there)
DeSmith (he was the best goalie in Pittsburgh in the 2018/19 NHL season and arguably the best goalie in the AHL in the 19/20 season)
Hill (he's the 3rd string in Arizona behind two band-aids)
Gustavsson has looked below average in the AHL. The main argument for him now is that he was good in the WJC a couple years ago. Ottawa has other goalie prospects in the same neighborhood (Hogberg, Daccord) and they used a 2nd rounder last year on Sogaard.
I like Annunen, DeSmith, and Husso the best for long-term upside. DeSmith, Husso, and Hill for short-term.