Hearthpwn

1- Life Tap

2- Fireblast & Steady Shot

3- Armor Up (But it is tier 2 with awesome synergy with hero)

4- Reinforce (maybe tier 3 with good synergy)

5- Totemic Call

6- Lesser Heal (Priest is bad)

7- Shapeshift & Dagger Mastery (tier 2 with synergy)

This is for constructed obviously.

For Arena

1- Fireblast & Lesser Heal

2- Reinforce

And others. Depending on your draft, any hero power save for Armor Up can go up to tier 2

In constructed: There is only one answer

In Arena, Mage is currently best -- because it denies the opponent board advantage one ping at a time, with no health cost like the druid's hero power.

However, I feel like Warlock is getting there with healing -- because the aggro curse is spreading to arena, and Warlocks can capitalize on their ability to draw cards at will and just ignore a mage's flamestrike, which mages will only get in ~33% of drafts after the expansion.

It has to be Warlock, but I like to compare them by how good they are on their own.  If there was a game with just hero powers Paladin would be the best - it beats shaman, rogue, warrior, priest, warlock and hunter, and it's on par with druid and mage