First, are you Beast Mastery? If so, you will want to bring Ancient Hysteria (Core Hound) if your raid does not have a Bloodlust otherwise, or Stats (Shale Spider) or Fortitude (Silithid) if your raid does not have these buffs.
Otherwise, the major buffs are Mastery (Cat, but brought by Paladin and Shaman), Attack Speed (Serpent or Hyena, but brought by Rogues, Unholy & Frost DK, and Enhancement Shaman), and Spell Haste (Sporebat, but brought by Elemental Shaman, Balance Druid, and Shadow Priest).
Crit is also strong, and some debuffs are good (Spell Damage Taken, Physical Damage Taken) and so on, but it just depends on what your raid is missing.
I personally have Cat (Mastery is one of the best buffs you can give to a whole party in dungeons), Serpent (Attack Speed is probably the best self-buff), Sporebat (for Spell Haste, since we sometimes lack this in our raid), Wind Serpent (for Spell Damage Taken, since we again sometimes are missing this debuff), and Boar (for Physical Damage Taken, since again we sometimes miss that one).
Sometimes when I solo, I have a Gorilla with me for the interrupt against Pandaren rares.
The Cat is just my general use pet. Mainly because I like the model (Sambas). Dragonhawk is for the 5% Spell Damage debuff and Sporebat for 5% Spell Haste buff which I find are the most common buffs/debuff missing in groups. Tailstrider is my main raiding pet when there is more than 1 target, as the 12% Armour debuff it does goes on every target within 12 yards whereas if you were relying on a Warrior or Druid to do 12% Armour debuff it would likely only go on 1 target. It also applies the full 12% instantly whereas a Warrior or Druid have to stack up their debuff with 3 GCDs. The Quillen is something I rarely use outside of LFR, sometimes when things go bad everyone is far too busy to brez or something so the Quillen allows me to do that if required.
I wrote for myself (and the hunter in our other raid team) a little app because buffs&debuffs have gotten mighty confusing in mop for 10m hunters.
Select the people who are nearly always there or will be for the night and then take the pets for things you might be missing that night so you can easily adapt (players leaving/joining or players who switch spec due to fight requirements). http://stijndp.be/online/wow/CelunasRaidbuffer1.1.zip
Requires .NET4.0. Uses Wowhead's buff&debuff data. Small note that warriors can't do both CS and BS for instance. Since we do BS, tell him to CS.
Does not contain virusses but I'm sure someone can download it for you and confirm. It's also very simple and minimalistic app so don't expect fireworks.
Spirit Beast (I might stable it so I can get something else useful)
Wind Serpent (Might be stabled as well if I can find room)
I guess I'm going to try to find a Hyena I like, I already have a Quillen but we run with a DK and Druid in a 10m so I don't think we need the res. I have a Waterstrider (the one from Dread Wastes) I can try to get a Ravager if I have room (my stable is full of challenge tames / rare pets)