If you must know here's what I found are effective against me Bastion players:

>Genji is usually a good choice, assuming that it's alone (which usually happens when good players reposition).
>If you can flank them as Reaper and get the first shot off, they usually die too quickly to counterattack.
>If you can get to a good position, Phara, Mei (ice shot), McCree, Hanzo, Genji or Widow (and perhaps Ana) can play peakaboo with them, killing or forcing them to reposition.
>Tracer's stand ult. bomb charges quickly and can get rid of a good camp.
>Junkrat's bombs are effective against any static position, and if they're protected by a Rhein, it can also wear heavily against his shield, or even shoot around it.
>If they're close enough, a Roadhog or Rheinhart can hook or charge them out of their turret formation.
>Even with the delay, Ana can hit a static Bastion with a sleep dart as long as she has LoS.
>Because DVa's defense matrix relies on time rather than health, it can grant a good few seconds for a DPS to come and shoot it up.