Hmm. Good question. I'm not a fan of many of either, mostly just Blaziken, Swampert, Decidueye from starters and Mewtwo, Xerneas and Necrozma from legendaries.
I'd have to go with starters. I know a lot of people like them, and if one of the reasons behind Dexit is to be believed, cutting legendaries for balance would make more sense than starters.