It is not possible to depend on one card to win, although others might think differently.
It really depends on what deck and what format you're playing. I have favourite cards, but they might not be what most consider 'the best'.
Correct me if I'm wrong MTGWarrior, but I don't believe he was asking for "one card to win". Also, just because it isn't viable to build a deck around a single card, it's sure fun as hell.
As for my favourite cards, here they are.
Triskaidekaphobia:
I'm a big fan of win conditions that are weird by nature, and this is one of those win conditions. I can't
recall just how many times I've killed myself with this card on accident, but you can't even be mad. It's just an all around hilarious card.
Tree of Perdition:
One of my favourites since it came out, because I realized immediately what this was meant to combo with. Obviously the Phobia-Tree combo is great, but in my current standard deck I use this card for not only that combo, but also a combo with
Soul Separator to set their health to 1 before I burn them with
Collective Brutality. Plus, it's got a massive butt. Your opponents will have fun
breaking through this defender without a kill spell.
Demonic Pact:
I wanted to make this card work so badly for so long. I tried it first in a Dimir deck with a bunch of bounce spells to bring it back to my hand before it killed me, while I slowly burn them with it, and eventually
Crush of Tentacles. It also went pretty well in my
Starfield of Nyx deck, since you could use it for resources and as a creature, then you can use its first ability to kill itself before it kills you and
Starfield of Nyx can bring it right back.
Harmless Offering:
First of all, it's adorable. Second of all, this is the card that made
Demonic Pact viable. Mnemosyne over there will tell you that you can't rely on one card to win, but this 2 card combo made it into top 8 of a Grand Prix. I don't really need to argue this adorable little guy any further.
Assault Formation:
Ah yes, my very first semi-competitive deck. Why wouldn't you want to have a bunch of big butts in play and suddenly have them turn into massive offensive creatures too? The best part about using this card was the look on my friend's face when I played
Shape the Sands to kill his
Archangel Avacyn. I believe his words were "Huh??? That's a card that exists? WHO PLAYS THIS SHIT?!". Good times...
Sigiled Starfish:
I don't have a very good explanation for this one. I used it in one of my first decks and I just fell in love with the little guy. I've tried to jam him into almost every blue deck I've ever made. Now I've started this beautiful trainwreck