What are the proper positioning for each character?

I'm about to be a dick here because this isn't what you asked but here goes:

I think I'm seeing a theme in your hero picks.

You're a battle buddy. A serious peeler who leverages gamesense to protect key allies.

If you don't already frequently play in a duo or 3-stack environment, I'd be surprised.

Can I assume Offtank is your 1st choice?

And here's where I get dickish:

You should drop Symmetra, Mei, and Ana from your main pool.

Take the hours you save and split them between maining Zarya / D.Va (map/teamcomp dependent), learning Road Hog, and flexing to Moira when your team already has two tank players and one or no healers.

You'll never realistically get to play DPS unless you're a DPS main. Dps ego-morality says you shouldn't even want to, it's throwing basically.

Maining off tank will teach you positioning.

/r/OverwatchUniversity Thread