My starter set consisted of
* A Dragon, because Dragons are awesome and defend well against Flying
* A Mech, because Mechs are great against Beasts which are common
* A Humanoid with a versatile Skillset because Humanoids defend well against Critters
The actual pets are not a great option for you because 2 are from the pet shop and one is from an achievement
Celestial Dragon (150 pets owned), [pet]Lil' XT[/pet] and
Moonkin Hatchling. This set did reasonably well (and all the pets are really cute
) and I used those 3 exclusively to level to 25. I needed more pets though to defeat the final two Northrend trainers, Cata and Pandaria.
Since he is starting out and will have to level the pets, get him pets that do well in the leveling process. The most common wild pet types are: Critters, Beasts, Aquatic and Flying. Later it is also important to consider the pets the trainers have of course. By then your friend should long have become addicted to pet battles
With the choices I am posting I'd mostly go with the "kill it fast" variety in the beginning. Going the slow defensive way would probably be frustrating for a beginner.
1. Versus Critters: For defensive version - the Humanoid - I must concur with the
Anubisath Idol, the other tradeable Humanoids just don't get close to this one. He can be slow though, so I prefer to use Beasts to kill critters when possible. Most tradeable Beasts are only uncommon, however for me the Beast stones appear most common, so maybe it would be feasible for you to obtain a rare
Feline Familiar or
Cornish Rex Cat for the fast killing of all those pesky critters. If you don't have a stone go for the
Sunfur Panda or a
Darkmoon Cub
2. Versus Beasts: Just kill them fast with a Mech. For a nice versatile Mechanical Pet I'd go for the
Clockwork Gnome, he is fairly easily available from the AHs and is rare already.
3. Versus Aquatic: If you have any Stones you are willing to spend there are a couple of really awesome Magic family pets to be bought:
Jade Owl a magic pet with flying abilities against those Aquatic types, however the risk of not getting the optimal breed for the Jade Owl is pretty large, so instead you might consider stoning a
Yellow Moth instead. There is also the (uncommon)
Enchanted Broom which has a very versatile Skillset. If you don't have a stone ready you might consider
Nether Ray Fry he only has one Flying ability ... the rest is a nice mix of different ability types. I have leveled him but haven't used him much at Max Level.
4. Versus Flying: For a Dragon with Magic abilities I found
Chrominius to be most easily obtainable.
The next types of pets I would get after that are an Elemental because Mechanical pets do occur in the wild and nothing kills them quite as fast as a nice sweep with some fire. I'd go with a
Pandaren Fire Spirit because I really like using
Flamethrower and
Conflagrate
Next on the list would be some Frog to deal with any Undead I find. If you can find him
Mojo would be nice.
Hope this gives you some inspiration ... have fun getting your friend addicted to pet battles