Winston excels in mobility and getting out of a bad fight easily. He also excels in what I like to call "personal combat arena".
What I mean is, through the use of his easy mobility to get to the other team's backlines, especially where Widows will usually lurk, he can hop in, plop a shield down, creating a mini space where he can fight the enemy without fear of getting chipped from other team members of the enemy. His mobility also lets him do a lot of work going against Mercys who are flying around to other team members, making taking down the medic a lot easier.
That is why Widowmaker's 2nd counter, besides another Widow, is a winston, because although his weapon is a very slow DoT, Winston has a huge HP pool, can shield up to create a personal area to fight in, and can easily jump back out should his HP get too low, then jump back whenever ready.