There's no single best one. There's the right pair for what you're doing — and the habit of reading both.
Pick one AI and you start trusting it. Run two and you start noticing — where they agree, where they split, where one sounds sure and the other hedges. The split is the useful part. So this won't crown a winner. Answer three questions and it'll hand you a starting pair.
Everything the picker draws from, in plain terms. All free to use without paying. Behaviour described in words, not numbers, because the numbers change every few months.