I think Ariana's voice is amazing. Is she on Mariah/Whitney level? No, but just because you're not one of the "best ever" doesn't mean you aren't incredible. I definitely prefer her voice to Christina or Leona or pretty much any other Mariah-wannabe (except Kelly).
In a technical sense Grande is certainly above someone like Katy Perry or even Demi Lovato. As has been said, that doesn't mean she's on the level of Mariah or Whitney, but among current relevant singers, she is certainly near the top in terms of technical ability. That's basically just fact. I'd put her up there with Beyonce. That doesn't mean Grande's timbre is pleasing or that her diction is great, it just means in terms of her vocal range and other technical aspects she is one of the strongest making hits.
Christina Aguilera hasn't been a relevant singer in at least a decade, and Leona Lewis basically only has 1 hit in the U.S. How has she been holding up in Europe? Kelly Clarkson is inconsistent in her output (i.e. she isn't seen as totally relevant/popular). All of that is to say I am not sure they are relevant comparisons for Grande.