First off, decide on your position. Do you setup at the enemy backline? Do you mix in with your teams front line (i.e. stand behind Reins shield and hack from there)? Or do you play defensively and stay close to your supports?
You can decide to switch between those positions any time but be aware that you cannot do more than one at once. You setup for very specific situations, and trying to cover a large area will cause you to spread yourself thin very quickly. Just choosing one general area has you bouncing all over the map already.
-If you decided to play defensively, watch out for Doomfist, Genji, Hammond, Tracer.
-If you decided to support the front line, that would be Zarya, Reinhardt, Mercy… uh… i don’t really prioritize Moira, since she’s not worth it (She fades if you try to hack her), but yeah, i guess her too if you get the chance.
-If it’s the backline, kill Zenyatta first… actually, even if you’re supporting the front line, go to the enemy backline and kill Zenyatta first before going after Mercy… Ana, Zenyatta, Widowmaker, Zenyatta, Torbjorn, and Zenyatta. Zens heal orb is a pita for Sombra.
Prioritize the ones that are about to attack your team, or have escape abilities. In a teamfight, go for Reaper and Mei first. Do not let them use their escape/stall abilities (Wraith and Iceblock) hacking them ensures that the enemy team will have less people in the fight very very soon. Zarya is also good as she will bubble anyone your team is focusing on. Honestly, Rein is pretty low on my priority list since more often than not, NOBODY uses his shield anyway

If you have the area setup, you don’t initiate the fight, you disrupt the formation before the fight starts. Harass the backline and make Zarya bubble the supports, make Ana use her grenade, make Reinhardt use his earth shatter, then teleport before you take significant damage and your team now comes into the fight with the enemy abilities on cooldown. You only hacked one enemy, but the rest are on cooldown anyway. Almost as good as an EMP.
Speaking of which, during the fight, focus on Ana. Even if she doesn’t die, you just have to keep her from healing her team. More often than not, it’s Ana that carries the team, keep her occupied while your team picks off her team one by one. You only need to keep her occupied for three or four seconds, long enough for a Tank to die.
Depends on how often your team uses the healthpack. If they don’t use it, then there’s no need for you to hack any at all.
The logic for the healthpacks is that you only hack if you come back to them faster than they respawn. So you DO NOT hack the mega healthpack the moment the game starts, it’s already there, why do you need it to come back faster?
So you can either spend two hacks on two healthpacks, which translates to 16 seconds of hack downtime, or just alternate between the two and save the two hacks for the enemy.
Remember that every hack is 8 seconds of downtime. That’s 7 hacks a minute at most, not counting the time spent running all over the map. If you maintain the four healthpacks usually available in each area, you’ll be down to 3 per minute. Not practical. Especially considering that you’re not actually hacking every 8 seconds.
Don’t rely on it too much, just use it when convenient, don’t bend over backwards trying to hack someone just because you think it’s her job.
When coming up to an enemy, you have two choices. Hack, or shoot. If you Hack, and you succeed, the enemy will have his abilities shut down by the time he’s aware of you but you will be fighting with him at full health, and you with a weak gun. If you shoot, the enemy will have a chunk of their health gone by the time they turn around, but their abilities will be intact.
Widowmaker, do not hack, just shoot, if she grapples away, follow her with translocator. Only hack if a teammate is about to engage her and you want them to stay together.
Ana, hack her grenade, Zenyatta do not hack, McCree hack his flashbang, Reaper do not hack if he just came out of wraith, Reinhardt do not hack if you’re behind him since he’ll be shot by your team if he turns towards you anyway (Works a lot more often than you think), Orisa do not hack you counter her by being able to run around her shield, Zarya hack her bubble, etc.