Impossible to answer, it all depends on YOUR priorities...
Fast Upgrade
-Most turboprop operators
-CHQ/REP
-SKW
-XJet
Better-than-average Pay/Quality of Life
-Horizon
-XJet
-SKW
-AWAC
-COMAIR (but they are in BK, so who knows)
Stability
-American Eagle (not too likely to get fired by AMR)
-A few others have been historically stable (Horizon, SKW) but who knows what the future holds?
Bad All Around
-Mesa
Fast Upgrade (but blacklisted for life and despised by all )
-GoJets
I'm sure I've forgotten some, no slight intended whatsoever