I mean, Widow is pretty much the poster-child for counters when it comes to Pharah so already we're just on completely different wavelengths.
Like, really it's not even about Widow but "Hitscan" counters Pharah and Widow is just perfect for it since she does so much damage in a single shot with hitscan. Ergo Widow counters Pharah because that's just how the mechanics work. Reaper was designed as the tank killer pretty much, so he does particularly well against the tanks especially those that don't have mitigation, like Hammond and Roadhog.