Hearthpwn

Best card Ever

Inner Fire depends on having other cards to have a big impact on the game, which usually is considered a bad card.
So no, Inner Fire isn't the best card, not even a good card.

Inner fire ,  and divine spirit and silver back  make  a 6 mana cost  8-8 monster  with taunt 

While you can come up with pretty crazy combos you have to think about card value in terms of its individual impact.  Does playing inner fire grant you an awesome play or does inner fire + x/y/z grant you an awesome play.  Primarily think of where it's most devastating say with divine spirit combos, if you cant guarantee that its making the kill shot on the play you can guarantee that it's getting killed next turn.  Best case scenario there you make a 2 for 1 trade but most likely worse which in a game with 30 cards blowing a turn and a chunk of your deck is going to get you kicked in the teeth usually.  

It's a mediocre card. It gives mostly 0 utility unless played in a combo priest deck, which is pretty one-dimentional and won't work consistently against any reasonable deck.