Out of the hundreds that I have played over 4+ decades, my favorite was Myztek Dryken Delerosh. Built as an AD&D Fighter, he was converted to 2E and then 3E/3.5E.
Born Dryken Delerosh, the son of a Rich Thayan landowner, Dryken was a real SOB. He was cruel and had no moral compass. In fact, he was even willing to steal from his father. So, one day, when a messenger was delivering a parcel to his father, he killed the messenger and stole the parcel. In it was a magical helmet which he was more than willing call his own. However, he did not know it was a cursed helm (headband in this case) of opposite alignment. He went from being a CE jerk to a LG ... well, still a jerk, but one that played by the rules and fought for the good of all. He ended up as a PC willing to make the hard calls for the greater good.
Around 6th or 7th level he was part of a near TPK. The party found out a beholder was threatening a nearby village, and went to intercede, despite being out matched. The other four PCs and a few henchmen were all killed/stoned, and Dryken was only alive because he'd been charmed. However, the charm was broken and he made a last minute vengeance play an threw his entire necklace of missiles in the mouth of the beholder. Boom.
We ended the game then and everyone rolled new PCs. However, the DM quietly gave me the option of continuing to run Dryken. As a human, he could Dual Class and start over, not using his fighter capabilities for a while. He took the name Myztek and learned magic to better fight against the Thayan threat. That dual class turned out to be over a year in real life, and the other players did not catch on (despite his surprisingly high hp total) until he reclaimed his weapons and gear.
He went on to become the central figure in Waterdeep Politics, and eventually became a prominent member of Waterdeep Society. He was retired after the PCs took on and destroy Szass Tam and Thay was liberated from the control of the Red Wizards. However, that DM still runs that campaign world and he and I chat about once a year to establish the goals for Myztek, and every once in a while he calls me to have me play Myztek when NPCs go to him for help. He is the central figure in a secret cabal of powerful figures that keeps artifacts out of circulation, so there is usually a reason to seek him out, either to learn about an artifact, or to ask permission to keep it and use it. However, that version fo the Forgotten Realms is pretty messed up now - no Time of Troubles, Spellplague, or other developments since the Grey Box ... but there was a war that devastated most of the Realms, cutting the humanoid population down by about 90%.