Great Wall is unquestionably the worst Magic: the Gathering card
The Great Wall of China is the longest wall in the world, a monumental testament to human engineering and a national symbol of China. Magic: the Gathering's Great Wall is none of those things.
A not-so-great wall
Great Wall has a 1.068 / 5 rating on Gatherer. It is widely regarded as one of the worst cards in the game. What troubles me, however, as a long-time Magic: the Gathering player, is how few people name it as the absolute worst. To me, the case is unambiguous. Great Wall is so bad that no card in the game comes even remotely close. This point is indisputable.
Duels of the Plainswalkers
Landwalk is a keyword ability that makes a creature unblockable if the defending player controls a land of a certain type. The flavor of this makes sense for most land types: a creature with forestwalk is adept at navigating forests without detection. Likewise with swampwalk or mountainwalk. An islandwalk creature sneaks underwater. How does a creature with plainswalk... walk? It's really good at evading detection in wide, open spaces?
For this reason, there have never been that many creatures with plainswalk or effects that grant plainswalk. There have only been 6 -- the last one was printed in 2008, and only 2 have been printed this millennium. The only one that was even remotely playable was Boggart Arsonists. It's also the only one that kind of makes sense flavor-wise -- presumably these Goblins burn a path in a field, preventing creatures from getting near enough to block. It only appears in 27 decks on EDHREC. For nearly everyone playing constructed Magic: the Gathering, they have never once played a single game in which Great Wall's ability would have any effect. I've been playing Magic for 15 years, and the only time I've encountered a creature with plainswalk was in Shadowmoor limited. Someone who has played 10x or 100x as many games as me has still almost certainly never found themselves facing a plainswalker. Great Wall does, effectively, nothing. It has no utility, dramatically less utility than any card ever printed, and is thus, the worst Magic: the Gathering card.
Doing nothing is worst than doing something that is usually bad.
One with Nothing
One With Nothing is a classic example of a "bad card". It's been described as Magic's "Best worst card", for good reason. Despite the fact that, in a vast majority of games, One With Nothing harms its user -- often substantially, this "bad card" happened to be a great response to a popular deck at the time called "Owling Mine", which forced your opponent to draw cards and punished them for doing so. In that case, discarding your hand was a good thing, and it ended up actually showing up in sideboards at a Pro Tour!
A card can harm the user in almost all situations, but if there are some situations in which that "harm" can be made into an advantage, the card can't be considered to be the worst. If a card is bad for you in 95% of situations but good for you in 5%, it's still better than a card that does literally nothing. You can always just refrain from playing a card that would hurt you, but whether you play or don't play Great Wall makes no difference. There are no games in which it will benefit you.
Other contenders for "worst card"
If you search "worst magic card", you'll find some strong contenders. Juju Bubble, Wood Elemental, Sorrow's Path, Razor Boomerang, Break Open, Fasting, Mudhole, etc. I won't go through all of them, but for each one, it is plausible to come up with a situation in which they are better than a blank card, or to think of a deck where the card may be strong in, or a deck that it may be strong against. And if not strong, at the very least, useful. Nothing even remotely approaches Great Wall's uselessness.
 Boggart Arsonists
 Magic's Best Worst Card