First post in 7 years, felt like sharing. May have been stated already, but just in case it hasn't.. I present to you, the Crit Chaser.
Devil's Sight - You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.
Darkness - Advantage on all attacks, disadvantage to be attacked, cannot be targeted by spells that require sight
Elven Accuracy - Whenever you have advantage on an attack roll using Dexterity, Intelligence, Wisdom, or Charisma, you can reroll one of the dice once.
Hexblade's Curse - Any attack roll you make against the cursed target is a critical hit on a roll of 19 or 20 on the d20.
Preferred build to obtain this setup is Half-elf Soulknife 3/Hexblade 5. Bump your Charisma to 20 and make your Psychic daggers your pact weapon so you can use them with Thirsting Blade.
It requires one turn to set up - cast Darkness centered on yourself, then use Hexblade's Curse on the biggest guy you can see. On each turn after, you can make 3 attacks - 2 with extra attack, and one as a bonus action. Because of Darkness and Devil's Sight, you have advantage on all of your attacks; but because of elven accuracy, you have "triple advantage" instead. Finally, you score a crit on a roll of 19-20.
Rolling 9 times, you have an (approximately) 18/20, or 90%, chance to score a critical hit each turn, which you choose to apply your sneak attack damage to. Feel free to take the other 12 levels in rogue, and just crit it very hard.