Free win tier
If you are good at these heroes, they're basically free wins. The entire game warps to be about you, and the game ends in draft if the enemy team doesn't dedicate their whole team to countering you.
S Tier (First pick/First ban)
Picks that will carry on all maps and play this iteration of HotS extremely well. All of them are extremely well scaling team fighters with good map pressure with the exception of Greymane, who can literally win a game with camp and map pressure alone.
A Tier (Power picks you can force)
Power picks that you can force on almost any map. Unlike S tier, they are counterable, but if you draft to support them, they can even play into their counters.
B Tier (Situationally Strong Picks)
In the right situation, all of these picks are strong, but none of them are picks you want to force. All of them are fine, but should be picked to complete a comp or punish the enemy comp rather than being forced on their own right.
C Tier (Underwhelming Picks)
Picks that seem to be underperforming. I haven't really seen any good reason to pick these heroes right now as it feels other characters do their jobs better.
If a character isn't shown anywhere on the tiers above, I don't feel like I've seen enough of them to get a proper judgement of their power level.