The Imp's DPS drops significantly when it runs out of mana, which is very quickly. Succubus probably has the highest sustained DPS over a longer period of time.
However, pet DPS isn't really much of a thing for warlocks. Usually Voidwalker is best for soloing because it's quite tanky, with a few select situations where Felhunter or Succubus may be better. Felhunter and Succubus can be very good in PvP.
For raiding you mostly either sacrifice your Succubus or keep Imp out in phase shift, depending on spec. There are a few encounters where you may benefit from another pet. In 5-man groups you can sometimes find some situations in which the other pets are useful.