If you're not BM, you'll get the most dps from a Ferocity pet with the right talents. As BM I can't confirm this but I think exotic pets (devilsaur-spirit beast etc) do slightly more damage then normal ferocity pets?
Different pets bring different buffs, so it can be important to choose the pet based on what buffs the raid is missing. For example if you have no shaman or mage, and you are dual spec BM/Survival - you'd probably be best playing as BM and using the exotic Corehound for Heroism.
Monkey is a cunning pet by the way, meaning it will do less damage in dungeons/raids then a Ferocity pet.. but could be good for PvP as Cunning pets have a few good talents for PvP, and of course the Monkey special ability
If you really want to be useful to your raid now, you need to start collecting pets. No one pet is the answer. You need to be prepared to be a swiss army knife of raid buffs. With regards to damage, the ferocity pets will all do about the same damage. The question is, which buff is your raid missing. Wolf is overridden by Feral Druid and a few others, Cat is the same as Horn of Winter, Rapter does sunders. None of these stack.
I usually take my cat for MM and spirit beast for BM because very few DK's remember to refresh horn of winter. Take my core hound and wolf as back up tho.
there isnt any right pet for raids anymore before it used to be a wolf cause of furious howl but now nearly every race and class can bring along the buff that your pet can provide so its what ever the raid is missing.
TBH it doesnt really matter aslong as it is a ferocity pet which does the most dps but i would stick to either wolf or cat if your not bm if you are bm then you can choose what ever you like
I usually take either my cat, if the buff isn't provided, or the wolf, it that buff isn't provided.
But otherwise, you can just take whichever ferocity pet you like.