Warlock.
Even though the very idea of playing a Druid gives me skin rashes (bathing IS part of nature, you bird-poop-on-beard weirdo! Stop going through my trash cans!) and I have a general dislike for shaolin-type Monks, those two classes seem to have well-defined roles, even if they could conceptually be covered by Clerics and Fighters.
Warlock, on the other hand, while I have no problem with the flavour, feels too much like something you could simply replace with a Sorcerer or Cleric subclass.
It could be just a matter of time, though. Monks and Druids were introduced long ago enough that they have become a stapple of D&D and have had time for a lot of material to be published about them. Maybe Warlocks are still too new. But as things stand right now, you're taking your creepy pact stuff off this island, mister.