Introduced in the original Dungeon & Dragon's first supplement, the Thief is a staple of fantasy as a whole, taking "cloak-and-dagger" in its most literal sense. In modern editions, the Rogue is a common sight for any party, second only to Fighters in popularity.
While they vary in presentation, from inquisitors to assassins, they boast the same skills: evasion, expertise, and an affinity for backstabbing. Here are some of the strongest Rogues seen in the D&D lore.
Spoilers for Dungeons & Dragons: Honor Among Thieves
5 Regis
As a Companion of the Hall, Regis is quite well known, adventuring with figures such as King Bruenor Battlehammer and Drizzt Do'Urden in the Forgotten Realms' most renowned adventurers. To list all his adventures would be an article in itself, but they include the typical fantasy shenanigans. He was raised as a rogue by a crime lord, then got bored soon after and ran off with the lord's magical ruby pendant. He was hunted down for a while, got captured and later rescued, captured again and later rescued, died, and was reincarnated by the goddess Mielikki.
Regis' tale is a long one and yet, he was only an 8th-level Thief – at least in 2nd Edition. He relied on the other Companions quite heavily, not believing himself to be a hero like the rest of them, and his repeated captures see him as the weakest of the group.
4 Erevis Cale
Originally known as Vasen Coriver, Eravis was raised as a member of the Night Masks, a thieves' guild that ruled the city of Westgate. After stealing profit from his jobs, he faked his death and fled to a smaller thieves' guild in Sembia called the Night Knives. Issues in alliances later rose, and after some violent complications, Cale was Chosen by the god of thievery, Mask. This in itself had some issues, and it ended up with Cale being killed by Mephistopheles, frozen in the Eighth Hell of Cania, then thawed out by his son who took his mantle as the Chosen of Mask.
That's the idea, anyway. The Everis Cale trilogy of books explains it in far greater detail, as books do. As far as power goes, Cale was a Rogue 8/Fighter 3 and a Shade, which gave him abilities like accelerated healing, leaping between shadows, and invisibility. Overall, he's one of the tougher ones and more than deserves a place on this list.
3 Forge Fitzwilliam
Found in the movie Dungeons & Dragons: Honor Among Thieves and its prequel novels, Forge Fitzwilliam is the archetypal rogue: wily, cunning, and liable to betray. He begins the story as a hustler, but as the story progresses, his skills are made evident, peaking as he betrays the party to become Lord of Neverwinter. He set out guards to arrest the rest of the party, re-introduced blood sport and set out a hot-air balloon with his face on it. The usual.
Forge sits at CR 8, or about 11th level as a player equivalent. While that's not much villain-wise, it makes him one of the strongest rogues in the series, though a fair part of his strength and influence is due to his party not knowing the typical tropes: if the rogue's not introduced brooding in a corner, they're going to betray someone instead. At least, usually.
2 Alatos "Ravenscar" Thuibuld
Here's an old one. Alatos (left) is found in the 1998 game Baldur's Gate as the leader of the eponymous city's Thieves' Guild. He keeps good relations with city officials while stomping out the competition, with his influence going all the way up to the city's Council of Four. In-game, he hires the player for some simple burglary jobs, though one of his thieves, Resar (right), may attempt to kill them afterward, depending on the quest progress. If the player survives, Alatos will spare them, occupied by the potential mutiny.
While leading such an influential guild is valuable, what makes Alatos truly strong is that he's in Baldur's Gate. If anyone wanted to kill him, they'd have to deal with THAC0. Jokes aside, it's a great game. Back on topic, Alatos is a mighty 19th level in 2nd Edition, making him one of the strongest mortal NPCs in the series, as far as humans go.
1 Jarlaxle Baenre
Jarlaxle is the leader of the Bregan D'aerthe, an elite mercenary company based in the drow metropolis of Menzoberranzan, though also seen elsewhere like the Sword Coast. He's featured in almost as much material as Drizzt Do'Urden, though readers may best recognize him from Waterdeep: Dragon Heist. As a skilled fighter, Jarlaxle is fond of the flashier, swashbuckling style of combat, though his efficacy with swords and daggers is enough to impress high-end assassins such as Artemis Entreri.
An interesting note about Jarlaxle is that he isn't a Rogue – at least, not technically. He was a Fighter in earlier editions, then a Skirmisher in 4e. The closest level equivalent is his CR 15 rating in 5e, including 19d8 hit dice which implies he's 19th level. Although, his access to perks like Legendary Resistance and a lot of magic items puts him well into the 20th range.
He's also a Rogue in Idle Champions of the Forgotten Realms, for whatever that's worth.