Define "best rated" here. Best rated by who? Anya's popularity in general is mindboggling, random parents on the street have watched Spy x Family when the only other anime they've ever watched is 2 episodes of Jojo and half of KnY. It probably helps that Spy x lacks a lot of "animeisms" for lack of a better word, very little uniquely Japanese cultural stuff, the environment isn't Japanese suburbia, the school isn't really a Japanese school, and the fanservice, while there, really isn't anything that puts off normalfags. Anya also actually acts like a kid for a shocking proportion of her screentime, unlike most lolis which are basically small adults or teenagers packed into a jewish or child's body. That's a pretty big plus for attracting an older audience who either had kids that grew up, currently have kids around the same age, or wish they could have kids.
On the other hand if we mean best rated by sadpanda then the moment Prillya 4 comes out it's not even going to be a competition unless some other series I'm forgetting comes swinging in out of left field.