"How well do they sing?" is kind of a golden snitch for me: it's hard to gain my favor without a solid voice or lose it with one. This year, Russia, Montenegro, and possibly Poland or Denmark are the only countries where "how well do you like their voice, Lavieenrose?" and "how well do you like the entry, Lavieenrose?" have extremely different answers. That being said, here are a few of my absolute favorites:
Serbia & The Netherlands (I put these two together since they're both representative of the big, soulful tones that I love)
Slovenia (probably my favorite voice of the year since it's so expressive)
Albania (Power. Vulnerability. For what else could I ask?)
Azerbaijan (I get both Broadway and rock vibes on the same notes, and his range is also very impressive)
Latvia (that last "your LOOOOOOOVE." 'Nuff said.)
Georgia (though my one complaint is that the song doesn't show it off well)
Malta (there's something very vulnerable when she goes that high, even if she misfires every now and again)
Iceland (there is a lot that she can do with her voice)
Finland (not for every genre, but for punk, certainly)