Best card: Hmm, either a Black Lotus or a Korean Volrath's Gardens, which has the ability "2, Tap a creature you control: You win the game. Play this ability as a sorcery."
As if it was significant, I will tell you that my favorite card (which doesn't necessarily make it the best - that means you, Zadok!) is Aluren. It's quite good too, but it isn't good to the level of the Korean Volrath's Gardens
I agree with Cateran Emperor's points about what the worst card must be. Hence, while Karplusan Giant and Cyclopean Mummy are both very bad cards (and I happen to think that the Giant is the worse of the two), the worst card has to be some enchantment (because that way only one or two color(s) can use it) with a weak effect. The worst card is probably Great Wall, despite people's points about Pale Moon. This is because of Great Wall's text:
Great Wall
2W
Enchantment from Legends
Creatures with plainswalk may be blocked as though they did not have this ability.
Since literally 1 - count 'em,
1 FRIKKIN' CARD* has built-in plainswalk, and I think 3 other cards (none of whom are used commonly) are capable of giving/getting it, Great Wall is only a little bit more useful than a card would be if it said "Sacrifice target non-green non-creature Blastoderm."
* That card is Righteous Avengers, a 3/1 plainswalk for 4W. That is up there too.