this is primarily a thread on warlock pets and their respective benefits/downfalls per situation. please feel free to add if you like. i am in no way a wow god.
firstly i will go over each pet and their respective buffs and damage abilities.
Imp- a small and fairly balanced pet. its abilities are fireball, phase shift, fire shield, and blood pact. Fireball is his general attack spell, hitting fairly decently, and with certain talents can be buffed to attack faster. Phase shift is an ability that makes the imp himself (not you) completely immune to all attacks to include AOE, unless he attacks. Fire Shield is a castable buff on party members that applies a damaging shield around the castee, a good way to help a tank generate just that little bit extra agro. and the largest reason an imp will be used, blood pact, when the imp is summoned he will increase the health of all party (or raid. not sure.) members by a certain amount. at 80 my imp will increase health by over 1k.
Voidwalker- the second pet a warlock will obtain. this is a tanking pet. he doesnt deal a whole lot of damage, but can take a beating. with abilities like torment which will taunt the target. making it more likely to attack the void walker. next he has the ability suffering which is an AOE taunt. it does the same thing as torment but to all enemies with in ten yards. next is consume shadows. when activated it will heal the void walker for a scaled amount depending on talent level. it also will increase your stealth detection. and lastly is sacrafice. you instantly sacrafice you void walker and in return you are given a shield that protects you for a set amount of damage or 30 seconds. higher levels will increase damage absorbed also with talents. as my spec when i sac my blueberry the shield will absorb 30K damage. through talents and what not.
Sucubus- the third pet a warlock will obtain. this is a DPS pet which produces higher damage than the imp but cannot take as much damage as the void walker. first ability this pet has is Lash of Pain, which instantly attacks the target for a certain amount of damage as shadow damage. through talents the cooldown on this ability can be reduced. next is lesser invisability, pretty much your pet becomes invisible for five minutes or until she enters combat. next is seduction which will seduce a target making the unable to do any attacks or spells for up to 15 seconds. a trinket can break this. if you are an alliance lock will of the forsaken will break this also. it is classified as a charm. next is soothing kiss. this ability will sooth the target making it less likely to attack the sucubus. the reason this pet isnt a good solo pet IMO.
Felhunter- the fourth pet a warlock can obtain. the felhunter is a caster counter class pet. with abilities that can spell lock, removed buffs from enemies, and debuffs from the warlock, and heal himself. first is devour magic which will purge one harmful magic affect from a friend, or one beneficial magic affect from an enemy. if the magic affect is devoured the fel hunter will be healed for a set amount increased by spell level. next is fel lintellect which increases the intellect of party or raid members by a set ammount increased by talents and spell level. next is shadow bite. an instant attack that deal set damage as shadow and is increased by the number of DoT you have on the target. in pvp this pet is best for affliction locks. and finnaly spell lock. instantly interrupt silencing them, and if used when casting,it will prevent that school of magic from being cast for 6 seconds.
Felguard (demonology spec only)- the felguard. there a demonology talent spec a warlock can obtain this pet. by far the strongest of all the pets with the highest base damage and dps through out. with abilities like anguish, cleave, intercept, and a passive ability called demonic frenzy. Anquish is and ability that will taunt the target, making more likely to attack the felguard. cleave is and AOE swipe that will attack three targets infront of the felguard. intercept is the same thing as a warriors charge, the fel guard will charge the enemy dealing base damage, increased by talant level, and stunning the target for three seconds. now demonic frenzy is a passibe ability that when the felguard attacks it will increase his attack power by 5% for ten seconds, the ability stacks ten times. all new attacks refresh the timer. with three points into demonic brutality it will increase his attack power by 80%. also with the glyph of felguard it will add 20% base attack power to him.
these are just some of the abilities of the pets. i will post another thread about the situations that each pet will be more resourceful than the other.