Saber beating the tar out of the Dementors and actually killing one. What makes it even cooler is that its the first thing he does after being summoned.
Saber vs. Archer
The second time Assassin humiliates Draco Malfoy is both awesome and funny: she effortlessly breaks free of a paralysis spell he puts on her and scares the hell out of the spoiled little brat.
The battle on board the Hogwarts Express. All of it.
Rin threatening Malfoy and punishing him for his bigotry.
Speaking of Rin, her fight with Assassin also qualifies as awesome, especially since she managed to hold her own long enough to draw Archer's attention, forcing Assassin to get out while the getting is good.
Lancer gets one in Chapter 4 when, after enduring a long and annoying period of listening to Bellatrix's pureblood supremacy ranting, he tells her how pathetic and foolish her bigotry is and, when she angrily tries to use the Cruciatus Curse on him in retaliation, he effortlessly brushes it off, grabs her, threatens her, and then tosses her aside like a wad of garbage and walks off.
Harry getting the better of Fudge with Saber's help and turning what was going to be a Kangaroo Court case into more of a simple interview.