Sorry I don't have Meiko up jere! I don't know what her voice sounds like and I listed them all by pitch (I hope it's correct, tell me if I'm not!)

I think Len, Luka's, or maybe Gakupo's voices sound the best.

Really its less on the Vocaloid and more on the editor. If the editor and tuner is good then the Vocaloid can sound super real and natural while on other hand if they are bad it can sound horrid and super artificial

I didn't hear IA's voice before because seeing her voice is very rare on songs
But i go with gumi since Echo is awesome and it sounds a little live like link

It really depends on how they're tuned, but I'd say Gumi has a pretty natural voice tone. She can sound pretty unrealistic too though.

It's between Gumi and IA. They both have naturally realistic voices, but the others can sound real too if tuned right.