Hektor and Achilles' fight. Both the boxing one and the proper one
Tenaka's return in Quest for Lost Heroes.
His ancestor Ulric pops in for a brief cameo during The King Beyond the Gate and it's every inch as memorable.
The Last Charge of the Avatars
Shannow clears nearly an entire town with a population of two thousand of its criminals by shooting one crimeboss in the middle of the street and stating at he will hunt down any criminal still in the town by morning. Two-thirds leave within the hour.
Shannow using Lucas to take the Sarento who became the Bloodstone back to the 20th Century just like he wanted. Right back to a couple of minutes before the Trinity A-bomb test detonates.
Argurios and the Eagles holding the Trojan Royal Palace against a much larger force of Mykene mercenaries and traitors
Jaim Grymauch coming to the rescue at Maev's execution Dying Moment of Awesome and using his body to shield her from the muskets
Really, Jaim Grymauch full stop.
Waylander (a quintessential Badass Normal) getting the better of Zhun Chao (a sorcerer of no mean skill) in the spirit dimension through intelligence, imagination, willpower, and a having balls of solid granite.
Rayster, wounded and missing an arm killing Lord Winterbourne with ease.
Rage proving time and time again he's probably the most deadly character in the entire Rigante series.
The Rigante defeating and killing Jarasay despite him accurately guessing and countering their strategy