If you want to give yourself a shot of winning you NEED to do the following. Otherwise it’s a soft throw.
1. Enemy Team has Double Sniper/One good sniper
You HAVE to pick Orisa and zone them out. No substitutes.
a) Sigma: Not good enough. No long range, unreliable shield
b) Rein: Not good enough. Sniper will kill him as soon as shield goes
c) Winston: Used to counter Hanzo. No longer.
d) Hammond: Maybe…if they know what to do
2. Enemy Team has a Moira
You HAVE to pick Moira in return. No substitutes.
3. Enemy Team has a shield tank
You HAVE to take another shield tank, or be a god like Hammond/Winston. Most probably your Hammon/Winston will not be godlike
4. Enemy Doomfist/Hammond/Lucio is Destroying Your Backline
You need a defensive Sombra.
Or if you won’t get a defensive Sombra, pick Brigitte. Not the best counter, but can disrupt them both if played well.
Other picks are viable ONLY if they’re good enough. Sombra however, can neutralize them as long as she stays behind her team and doesn’t do stupid stuff like flanking etc.
Mei: Might not be good enough
McCree: Might not be good enough
Reaper: Might not be enough
Hog: Might miss hook
Remember: If you don’t follow these win conditions, you WILL lose. And for god’s sake, do the right thing as soon as you realize the match condition. Not 10 seconds before the match ends. So many times I’ve seen people bite the bullet and do what needs to be done only in the last few desperate seconds of a match.
Save yourself and your team. Switch at the start.