Technically, the highest damage character is plague doctor....

[xxdareyxx]

Upgraded cause of death + DOT team + enemy with lots of DOT stacks means she can do up to 40+ damage everytime. My go to DOT team is runaway, plague doc, grave robber, and flag. Once had the DOT heavy stacked on stage 5 boss and plague did about 120ish damage on the boss.

[Null Winter]

I've hit over 200 with a critical revenge-boosted Chop on an Eldritch enemy with Leper.

[RopeDrink]

Hitting hard after setup is one thing, consistently hitting hard is another. I would rather someone hit for 15-35 x 2 every round without preamble, compared to spending all day pumping DoTs into a target - which, by themselves, will already kill it - and then dispelling those DoTs with one raw damage hit.

TLDR: 120 damage isn't much, and if it takes a whole team of setup to achieve, then its not always worthwhile.

[lukeliu]

Yup, the Plague Doctor is pretty much the best damage dealer in DD2.