The most absurd damage dealing build I've concocted is Thri-Kreen (from when it was still UA so idk if it's been changed at all) Cavalier Fighter 5/Assassin Rogue 5 with Great Weapon Master. Use your main hand to wield a rapier and a shield, and your secondary hands to dual wield shortswords. Let's assume +4 STR. This is kind of a lasting thing you can continue for 3+ rounds, assuming flanking or something else to get Sneak Attack each round too
Round 1: 1. If disguised (using Chameleon Carapace) before combat, you have a higher chance of getting a surprise round, which means you go before everyone else in combat, which means you get Assassinate on as many creatures as you'd like and every hit is a crit. That's step one.
2. Using Great Weapon Master on every attack (which should be fine because you have advantage on every attack this round), attack twice with the rapier, one of which has sneak attack, for 4d8+6d6+8+20 for your action. One of these gets a free Unwavering Mark
3. Action Surge, do the same thing again, still all crits, for 4d8+8+20
4. Use the Great Weapon Master special bonus action attack to make another rapier attack, still with advantage and GWM, for 2d8+4+10
Round 2 (assuming no crits for any rounds after round 1): REPEAT UNTIL YOU RUN OUT OF UNWAVERING MARKS 1. Two GWM rapier attacks, one with sneak attack. 2d8+3d6+4+20
2. Unwavering Mark bonus action rapier attack, for another 1d8+4+10+5
Round X (for when you're out of resources (Unwavering Mark, Action Surge): 1. One rapier attack for 1d8+3d6+4+10, and one shortsword attack for 1d6+4+10
2. Bonus Action Two Weapon Fighting attack with the shortsword for 1d6+4+10
The most absurd damage dealing build I've concocted is Thri-Kreen (from when it was still UA so idk if it's been changed at all) Cavalier Fighter 5/Assassin Rogue 5 with Great Weapon Master. Use your main hand to wield a rapier and a shield, and your secondary hands to dual wield shortswords. Let's assume +4 STR. This is kind of a lasting thing you can continue for 3+ rounds, assuming flanking or something else to get Sneak Attack each round too
Round 1:
1. If disguised (using Chameleon Carapace) before combat, you have a higher chance of getting a surprise round, which means you go before everyone else in combat, which means you get Assassinate on as many creatures as you'd like and every hit is a crit. That's step one.
2. Using Great Weapon Master on every attack (which should be fine because you have advantage on every attack this round), attack twice with the rapier, one of which has sneak attack, for 4d8+6d6+8+20 for your action. One of these gets a free Unwavering Mark
3. Action Surge, do the same thing again, still all crits, for 4d8+8+20
4. Use the Great Weapon Master special bonus action attack to make another rapier attack, still with advantage and GWM, for 2d8+4+10
Round 2 (assuming no crits for any rounds after round 1): REPEAT UNTIL YOU RUN OUT OF UNWAVERING MARKS
1. Two GWM rapier attacks, one with sneak attack. 2d8+3d6+4+20
2. Unwavering Mark bonus action rapier attack, for another 1d8+4+10+5
Round X (for when you're out of resources (Unwavering Mark, Action Surge):
1. One rapier attack for 1d8+3d6+4+10, and one shortsword attack for 1d6+4+10
2. Bonus Action Two Weapon Fighting attack with the shortsword for 1d6+4+10
:)