Doubt you'll get a consensus. I don't get to watch them play, but from what I have gathered Valiev and Loov would be most ready to step in if needed, but neither has looked great this year, and aren't surefire NHLers (esp Loov). People have been saying Dermott may be at the front of the pack by next year, and Nielsen probably has the highest ceiling, but also the most to work on.
Nhl readiness - Valiev, Loov, Dermott, Nielsen
NHL Potential - Dermott, Nielsen, Valiev, Loov.
Other 2 are career Ahlers.