The second episode of The Hidden One Year War featured the failed prototype YMT-05. It's moment came when it manages to take out six stolen Zakus single-handedly. The panic that ensued when it changes to a half tank, half mobile suit form was also priceless. Sadly, it ended with the death of the test pilot as well.
The Hidden One Year War: After being shamed in propaganda, Major Jean Luc Duvall manages to pull this off, along with a Heroic Sacrifice during Zeon's escape from Odessa. He almost single-handedly defeated the squadrons attacking the stranded escape pods, then he lured a newly arrived GM team into a race where the sheer speed of the chase blew them up. He was even able to give a Final Speech to the crew of the Jotunheim shortly before dying.
Erwin Cadillac and his team in Apocalypse 0079, a trio of Zeon cadets sent out in worker capsules hastily refitted with weapons, take out a full squadron of Balls all on their own. One of them by whacking it with its own cannon (which Erwin had previously ripped off).
During the Battle of A Baoa Qu in Apocalypse 0079, Engineering Lieutenant Oliver May, despite having no previous piloting experience, uses the Big Rang Mobile Armor and slaughters E.F.S.F. forces left and right, until got to the point where enemy pilots thought he was Char. This is helped along by the fact that the Big Rang was red.
Not to mention that Oliver is the only test pilot to survive the series.
In the first episode of MS IGLOO 2: The Gravity Front, when Lieutenant Ben Barberry fights and defeats a Zaku II on foot using a giant pothole and a guided missile.
Shortly after that was awesome as well, when the third Zaku burst from the ground, the camouflage netting caught on it's back made it look exactly like what The Federation soldiers called them, Shinigami. And then, the Lieutenant draws his pistol and starts shooting at it knowing full well he was going to die.
In the second episode of MS IGLOO 2: Harmann Yandell and Leiban Sura and their allies taking down four Zakus in tanks, despite the loss of the entire squad except the aforementioned two characters by the end of the skirmish, killing one of Zeon's most renowned aces in the process.
From the third episode of MG IGLOO 2: Guntank charge!