A single-classed Rogue (Thief to get extra bonus action) gets the most out of their sneak attacks (keep in mind that if you multi-class, you don't benefit from the increased sneak attack damage that goes up every few levels - you're basically throwing away free damage) and three attacks per round. [Not saying you must single-class, just saying make sure you take everything into account]
Dual wield hand crossbows and short swords - especially those that increase your crit range and do free extra damage on crits - makes Rogues extremely dangerous. Combine that with the ability to go invisible (Rogue skill at higher level) or magic items, expertise allowing you twice your proficiency bonus on certain skills just make them even more amazing.
If you don't have advantage, move another character into melee range with your target, then switch back to the rogue for sneak attack against that target (if they share initiative). In one playthru I used a monk with high movement rate to just move around the battlefield getting next to targets so the Rogue could annihilate them with sneak attacks.
Special arrows can crit and do allow sneak attacks, and the mindflayer ability that grants a free crit combine for high damage numbers.
There are many large fights that you can actually solo with a sneaking rogue kept at distance.
Not saying you can't multi-class with it, just make sure you understand what you're giving up. Many people combine Gloom Stalker (Ranger) with Rogue, but this means you actually lose abilities (they each gain some of the same abilities, so you aren't getting the full benefit of a new level). Assassin is good, but only on the first round of the fight. Any fight that drags on longer than 1 round, the Thief will out-perform the Assassin.
Keep in mind that multiple attacks beats fewer attacks, even if those multiple attacks do less damage (each attack gives an opportunity for a crit, sneak attack, elemental damage from magic items/spells/abilities, etc). And multiple attacks protect against bad dice rolls resulting in a miss (no damage inflicted). Be careful with Sharpshooter. There's a saying, "You can't miss fast enough to win a gun fight." Sharpshooter is only a good idea if you're fighting something with a low enough AC to still hit with a 25% penalty on your attempt. Against big baddies with really high ACs, turn off sharpshooter. I would argue that Crossbow Expert and get to a 20 dex is better for a rogue than going with Sharpshooter (unless you already have a 20 Dex and Crossbow Expert, then go ahead).
