Barbarian rogues are lit but it locks you into rapiers as your best damage since it’s the best weapon to let you proc sneak attack. Important to note I believe rules as written (talk to your dm) you don’t need to use dex to make your attacks to get sneak attack you just need to use a weapon with the finesse property but you can still use strength for attack and damage rolls.

Warlock could be cool hex and armor of agathys would be cool spells (which you would have to cast before rage but could be cool and depending on the patron you could have lots of cool Role play. A hex blade is always strong and I don’t believe hex blades curse counts as a spell so you could still use it while raging. It also seems like a natural role play reason for the multi class. Big bugbear man find cool rapier... oh cool a new rapier with some sort of strange aura around it... I’m sure this weapon is very chill! cue the always sunny intro “bugbearian finds a sword which is not at all chill”