You probably would of guessed that my favorite class is the Warlock. They can summon demon pets and in my opinion are one of the most complexed classes to do. They have shed loads of Warlock only quests (which some of them can be annoying eg. epic mount quest) and they are also good in PvP with the ability, Fear.
To be honest with you the imp isn't the best pet, in terms of usefulness its pretty good in groups, with the stamina buff and Fire Shield. The Voidwalker is easily the more superior demon when you are soloing. So really there isn't a 'better' pet. They have all good characteristics.