2 Tanks, One strong healer, and three damage.

One of your two tanks needs to be super defensive, this is where Reinhardt comes into play. Just stand their and hold up your shield, only attack when you're 100% sure you'll land the hit.

The other tank needs to be offensive. Zarya shines here, if they attack you while your shield is up it causes your weapon to deal more damage. With heals on you after your shield goes down you're able to take on anyone in the game and out damage them. Plus the fact she has a ton of HP means she'll rip through their front line pretty easy. D.va is also really good as a secondary off tank.

You should be healing with Mercy or Lucio. Zenyatta is decent but could use better heals and also has low hp and gets taken out pretty quick. I suspect they'll buff him eventually. It's best to use Lucio on maps where there are several spots for someone to fall off. His secondary attack pushes enemies far enough back you'll send them flying off the map.

The offensive guys just pick who better fits your play style. Lot of counter picks depending on who the other team has. Genji can reflect everyting Bastions sends at him killing him almost instantly if the Bastion is dumb enough to keep shooting. McCree flash bang and fan the hammer attack is one of the deadliest combos in the game, pretty much an instant kill if you can pull it off.

Interested and seeing what will happen once ranked gets released. I'm glad they're holding off until late June so everyone can get comfortable with the game first.