User blog comment:Spartian300/Why Titans are the best class/@comment-24057866-20140918155744

Titans are best in PvE because they have the highest amount of health. This not only allows them to survive situations that Hunters and Warlocks would've died in, but also allows them to deal more damage before needing to hide to recover. You could argue that Sunsingers can self-res, which helps protect a fireteam from wiping in a Darkness area, but Titans are less likely to die to begin with.

Hunters are best in PvP because both of their supers are way overpowered.

The only reason to choose a Warlock would be simple preference. I can't think of anything that they can do that one of the other two classes can't do better.