Gupsparkle Best - Svirfneblin Wizard
In a Curse of Strahd game he learned the secrets both to becoming a vampire and to becoming a lich. At the end of the game he decided to become a lich.
His close friend in the party was a paladin, and the interaction between the two as Gupspakle's alignment changed - due to bad choices, unfortunate circumstances, and adverse luck - from Neutral Good to Neutral Evil was epic!
During our next campaign the player who had played the paladin now played a goblin warlock whose patron was The Gupsparkle.
Brevis Facultus - Human Sorcerer
In our Out of the Abyss campaign I had to miss a session and sent the group a strategy guide for how to play Brevis as they planned an ambush on 2 demon lords. The feedback was great with one of the players saying it was one of their most memorable sessions as Brevis took out both demon lords and most of their minions almost single-handedly by following the planned strategy. It's a crying shame that I missed it.
... and there are so many more memorable moments!