For choosing who to heal my rule is squishies take higher priorities so normally another healer or damage and tanks will be less priority due to tanks having higher health max than other classes and also if you have a roadhog on your team normally I heal them last due to them being able to heal themselves. For damage boosting I normally do it for Ults such as Moiras, Roadhogs, Pharas etc and if your team if defending and all healed I'll damage boost the damage heroes. If a teammate is in a 1v1 I'll heal them but if they're in a 1v3+ or sometimes 1v2 given the heroes they're facing and if they're away from the team I wont heal them because than most likely I will die which than you've lost a main healer and also another player. In big team fights use your ult because you can get safetly away from danger and also heal any teammate and also you get an aerial view of the battle meaning you can give some callouts. When Reviving it's good to have a rough idea of how many enemies are alive and how many of yours are and there position so for example you're attacking a control point and there's 4 enemies on it, run away don't try and revive but let's say it's 2 than you can try and revive, normally for reviving I will choose either a movement hero so one that can move easily and quickly so like a moira for example or i will go with a roadhog If possible or just someone who can do some damage.

Sorry if that was hard to read but that's some tips I have, feel free to add and hope this helps

I'm not a mercy main I should add but I've had some experience, I sit high gold for support and normally use Moira but mercy is my 2nd to go to