15 red mana is likely enough to take out an opponent with a Deathforge Shaman, while infinite white mana turns Lightkeeper of Emeria into an infinite life engine
While perhaps different now thanks to the London mulligan rule, in the past, people have calculated that your odds of winning when you mulligan to six (so starting with one less card than your opponent) is 40%, which might not sound like a lot, but considering that the odds are 50/50 with each player starting with an equal number of cards in hand, it's actually a 20% shift in your chances of winning the game
Casting Cost: Card Text: Whenever you play a land card from exile or cast a spell from exile, you gain 2 life
Its clear to see why these are considered the best, but that power comes at a high mana investment and sometimes, in the case of Winds of Abandon, a downside
Angel of Despair Deathpact Angel Drana and Linvala Firja, Judge of Valor Liesa, Forgotten Archangel Liesa, Shroud of Dusk Magister of Worth Selenia, Dark Angel Seraph of New Phyrexia Seraph of the Scales