Hypothetically, if you had 30 of the same card in a deck, what would be the best deck? Or would there be no one best deck because of a rock-paper-scissors effect? I'm sure this has been discussed on other Hearthstone forums but it's a fun puzzle.
Here's the all-around strongest I can think of:
30 Soulfires.
Because of life tap, starting turn 2 you can draw soulfire, play soulfire, draw soulfire, play soulfire, for 8 damage every turn.
As player 1, on turn 1 you have 4 soulfires in hand and can play 2 of them, so every turn of the game you do 8 damage. As player 2, on turn 1 you have 5 soulfires and 1 coin, so on turn 1 you can: coin, soulfire, soulfire, soulfire, tap, soulfire, to do 16 damage. Then 8 damage on each subsequent turn.
Absent opponent healing, that gives player 1 lethal on turn 4 and player 2 lethal on turn 3.
Counters: Because the soulfire deck has a per-turn damage cap, decks focusing on healing have hope, but they need to be very fast.
Healing touch is a certain counter, because beginning on player 1 turn 3 or player 2 turn 2, you can restore 8 health once per turn, completely negating the soulfires. If the warlock chooses not to soulfire then you win easily via hero power plus fatigue. If the warlock collects soulfires in hand, the most burst possible is playing 6 soulfires in a turn for 24 damage, not enough.