Jérémie entering XANA's virtual trap in "Ghost Channel".
In the episode "False Start" where XANA first attempts to straight-up conquer the world by materializing a bunch of Kankrelats to the real world, forcing the team to literally fight their way through a small army to get to the factory, without their powers. Jim soon joins in the fight and holds the remaining army off with a nail gun the modified previously, nearly performing a Heroic Sacrifice before they were able to activate a return to the past.
In the episode "Maiden Voyage", Jérémie climbing across the dorm roof, despite his lack of athletic ability and the fact that he's about as scared of heights as Ulrich is.
Jérémie pulling The Plan on XANA in "Franz Hopper" and just standing there smiling about it.
In a similar vein to the Jim example above, Yumi's dad decking out in samurai gear to take on the materialized Krabes in "A Bad Turn"...even though Ulrich is himself doing just fine against them with a metal pipe.
Another scene from the same episode. Yumi's been shot by the Krabe, her family is watching as it charges its laser for a final blow, Yumi's mom is crying out, and as the Krabe fires the shot, Takeo's blade comes out of nowhere, deflects the shot, and shows Ulrich standing right there as he says, "Make my day, shrimp." Awesome indeed.
Same episode. Odd and William's plan to take down the Krabe in the factory: tie a rope onto the backseat of a small car, tie the other end to the Krabe's leg, and then drive the car into the elevator shaft while dragging the Krabe along. And the plan actually worked.
The new shield program beating the Scyphozoa in "The Lake" is impressive. Mostly because it's the single most decisive defeat dealt to the thing throughout the series.
What; no mention of Yumi managing to snap William out of XANA's control for a few minutes? It's worth noting that she was alone on Lyoko at the time.
In one of the early Season 4 episodes, XANA takes control of William's clone (yeah, having just the real thing wasn't enough) and it gets to the point where he has Ulrich and Sissi cornered on the roof of the school. Sissi, in panic, throws a tile at clone William, which beams him in head and causes him to plummet off the roof. Damn, no wonder Sissi's always the Action Survivor!
In the season four episode "Double Take", Ulrich tries to crash the Overbike into William and the three Tarantulas with him. William just Super Smokes up to it and, using his BFS as a ramp, jumps it over himself.
Despite it helping XANA, Aelita erasing the Ice Sector in "Sabotage".
Odd in "Bragging Rights", defeating William and destroying the latest Supercomputer of the latest Replika.
Jeremie diving into the pool and saving Aelita in "Deja Vu".
Ulrich calling out his dad for his unreasonable expectations and insulting Ulrich's friends and then flatly informing him that he's done listening as he leaves the room when the rest of the team needs his help.