Ed's Mart. While it may not have seemed like a big deal back in season 1 (when it aired), it becomes this in hindsight due to being the only scam that not only completely worked, but also (assumedly) turned out a good profit for the Eds.
They also seem to be making a successful run out of their pet store at the end of "Dim Lit Ed", albeit much to Double D's disgust.
Double D: I'm surrounded by idiots.
Eddy: Whose complaining?
At one point, Double D and Eddy use a puppet show to with a couple of Ed's toys to make Ed happy. Eddy gives up immediately and Double D tries to tell him off about it. Ed pulls the string on the back of his monster toy while Double D isn't looking. Cue the toy SHOOTING A FIFTY FOOT FLAME AT EDDY AND DOUBLE D. Eddy's reaction sums it right up.
Eddy: Nice toy.
Ed and Edd overcoming their fears and putting up a convincing facade to scare the Kankers in "A Twist Of Ed".
In the episode where Ed is in a bad mood due to a pebble in his shoe. He actually makes Sarah back off. (pictured right)
Sarah:Ed! You can't kick me out, Mister! I LIVE HERE TOO! Ed:SO MOVE!
Even more awesome, though, was after Ed had scared off Sarah and Kevin, Edd and Eddy didn't even CONSIDER running away. All they did was try to get Ed back to his former self. Their lives were quite possibly on the line, and they STILL basically just stood there and took everything Ed threw at them, all in the hopes of cheering him up.
Ed winning a spelling bee against Edd, even if it was through the words "ectoplasm" and "gravy".
While he's usually The Load or The Millstone, he has his heroic moments, like saving Double D twice in "Don't Rain on My Ed" or saving Kevin from the Kankers in "Dawn Of The Eds". And in "Momma's Little Ed" he's able to hold the Kankers off by trapping their heads in a measuring cup: this is one of the few times the Kankers get some comeuppance.
In "O-Ed Eleven" Ed, of all people, manages to puzzle out a map belonging to Eddy's mysterious older brother. Of course, he's still Ed, so he tries to explain it thusly:
Edd: Ed, how on earth did you do that?
Ed: Because I am a brother, and Eddy's brother is a brother, and Eddy is a brother to Eddy's brother as a brother I am.
Double-D has created many, many awesome things out of cardboard and other junk, but probably the best was the Thingamajig, the technological equivalent of a genie's lamp that spat out whatever it's user wanted, including things bigger than the Thingamajig itself. And Double-D acted as if creating more than one would have been no big deal.
And let's not forget the PLANE he built a season later. And that scanner thing he used in "Luck of the Ed."
In "A Case Of Ed", Ed and Eddy have been alternating between tormenting a grounded Kevin, and tricking Edd into thinking he had a fatal disease. Both Edd and Kevin get their revenge when Kevin gets ungrounded early for good behavior, and then, when Ed and Eddy lock themselves in Eddy's house so Kevin can't get at them, Edd "accidentally" drops the spare key to the house, allowing Kevin to get in and get his revenge, watched by Edd who proceeds to list the symptoms of what can only be known as "Karma-itis".
"Kevin's Justified Pummel Disorder. Symptoms are bruising of the eye, followed by a sore rear end and a rapid decrease of hot air from an inflated ego".
Parodied in The Day the Ed Stood Still, when Edd, stands up to Ed (who believes he's a monster) and if it weren't for the fact Edd knew it was Ed and not an actual monster, this would probably count...at least, that's my opinion.
Well, this is Ed, the individual who lifts houses like it's nothing. So, whether or not he's really a giant monster, Ed is in that mindset, so for all intents and purposes, he is genuinely threatening in that he's sticking people to walls, planning to do god knows what to them. So, I think it counts.
In "Avast Ye Eds", Edd plays a steel stringed guitar perfectly...while reading a book...and playing the guitar with his feet!
In "Pain in the Ed", Eddy wants to smash Ed's violin because Ed's a Dreadful Musician, but Jimmy is watching with specific instructions to tell Sarah if it happens. Edd suggests Making It Look Like An Accident, so he set up an extremely elaborate Rube Goldberg Device that included the precise reactions of the kids of the cul-de-sac themselves as elements of it...and it worked perfectly, including the last little bit he added to save the violin.
Anytime he snaps. For example when, in "X Marks the Ed", he angrily kicks the kids out of his house because they were making fun of Eddy. And who can forget the scene between him and Sarah from "Is There an Ed in the House?"
To clarify, Edd, who has gone through a series of coincidences that make him look like a bully, just got beat up by Jimmy of all people and the Kankers come in to give him and the other Eds the kissing ending they usually get, until Eddy screams at the top of his lungs the quote above to get them to back off. And they do!!
Adding onto the above moment:
Kankers stand there stunned for a few beats, then they do.
Ed (in awe): Good one, Eddy.
In "Sir Ed-A-Lot", after being pushed to his limit playing "princess" with Sarah, he gives her a taste of her own medicine by proclaiming himself king and locking Sarah in her own "cage". Now Eddy has the chance to have the last laugh while Sarah goes stark raving mad, screaming like a madwoman.
It may have been profoundly stupid, but it said something about how damn durable Eddy is when he won't back down in competing with Rolf for the Hairy Chest of Resilience Badge in "The Good, the Bad, the Ed." He took a RIDICULOUS amount of damage in that episode and still wouldn't quit. Determinator, indeed.
A minor one, but Eddy manages to run so fast in "Don't Rain on My Ed" that he manages to cause the pavement to crumple up beneath his feet like a carpet.
Just before the end of 'Ed, Ed and Away!', Eddy takes on Sarah and Jimmy for the balloon, and wins. Bearing in mind that Sarah can usually decimate all three Eds without effort, Eddy willingly vying for the balloon in a physical confrontation shows a certain level of determination beyond his usual self-preservation when Sarah is involved.