IA's my favorite vocaloid (especially low-tuned IA), but like
>>8, I tend to go by producers. From the top of my head, I'm a fan of MikitoP, SuperCell, Orangestar, and BarrageP. As for vocaloid tuners who cover songs, Cillia, Osamu, and GrandMasterFlames159 are my favorite.
My favorite vocaloid song right now is Sayoko, mainly IA's version shown here:
https://soundcloud.com/kamilia/sayoko-ia
https://soundcloud.com/reversereality/ia-sayoko-vocaloid-cover