At the moment, I have seen Roadhog picked a lot. He seems to be worthwhile, and if you're decent with him, you can do a lot of damage to the enemy team. You can pull someone out of co-ordination and then kill them.
Try to isolate enemies and pick off the squishies first.
If you see a Rein charging and you notice your healer has been pinned. HOOK THAT BITCH
Use your healing as both a disengage and also to obviously restore health, it works amazing. Also, don't be afraid to use it when enemies are nearby because you can still move whilst healing and you take 50% less damage, so you can actually survive a D.va ult as long as you have over 500 health at the time.
Don't be scared to engage and try to hook an enemy. If you miss, it doesn't matter, just fall back into your team a little bit and attack with your right click until you get the chance again. Always. Be. Hooking!
That's all I'm really gonna give as a starter, but the best way to get good at him, is just practise. And tbh the best place to practise with him, isn't the practise range, it would be in either QP or vs AI.