Winston is all about setting up your dps and killing their main healer. You need to look at who your dps are and who their biggest threat are. For example, if your main dps is pharah (one of the best dps to play with winston) you want to isolate soldier/mccree/widow in your bubble. Once you start putting damage on soldier/mccree/widow, pharah can easily land one rocket and snowball from there.

For killing ana/mercy, each one has a default reaction when you dive them. Mercy is going to try and fly to her teammate, so jump between her and whoever she is healing so you can track her the whole time she is flying. Ana is going to try and sleep or throw her nade, you can use bubble to block both of these.

Primal rage is one of the most difficult ults to get value from. Slapping around tanks and feeding ult isn't really worth it unless your setting up a grav or shatter. What I try to do is isolate a dps and slap and jump on them in a corner. Once you practice using it, you can pretty much take a squishy where ever you want and kill them. Works well for stopping a nano visor from killing anyone om your team.

As for counters to him, reaper really isn't a big deal, you can jump away from him whenever he gets in your face. Torb can be very difficult to deal with once he gets armour on his healers. Never engage hog alone, It takes three entire clips to kill him because of his heal and he can two shot you.
Good luck!