Honestly, when Trainor’s not trying too hard to be quirky or leaning too hard on her schtick, I think she’s the best of the bunch. Listen to her on Brett Eldredge’s cover of “Baby, It’s Cold Outside” that was released recently.
Grande is good, but her horrible enunciation and breath control is hard to listen to. Lovato has her moments, but she doesn’t have much in the way of subtlety and it doesn’t take much for her to sound shrill.
Swift and Gomez are both very bad. I’d put Gomez a hair ahead because her production fits her voice (she’s more producer conduit than singer) and she doesn’t try to do much out of her range. She’s more pleasant to listen to than Swift, who I think thinks she’s a better singer than she actually is.