It's not really that great, but Ranger 3/Monk 2/Barbarian 1 lets you use Horde Breaker, Flurry, Rage, and Hunter's Mark all together for four attacks per turn (with a ki point) with +1d6+2 damage on each. Rage, Ki points, and Hunter's Mark are limited resources, though, and a straight-up Monk 6 can flurry for four attacks anyway.
Add in another Barbarian level for Reckless Attack, plus two levels of Fighter for Action Surge and it starts looking like a fair bit of damage, but at that point your comparison is to a level 9 Battle Master Fighter or Frenzy Barbarian, and those two can just crank out the hurt all day. I haven't run the exact numbers but I'm guessing that the multiclass mess basically has to nova just to stay competitive.
Add in another Barbarian level for Reckless Attack, plus two levels of Fighter for Action Surge and it starts looking like a fair bit of damage, but at that point your comparison is to a level 9 Battle Master Fighter or Frenzy Barbarian, and those two can just crank out the hurt all day. I haven't run the exact numbers but I'm guessing that the multiclass mess basically has to nova just to stay competitive.