darkmoon tonk is a must have battle pet, as he just does insane damage...http://www.warcraftpets.com/wow-pets/me ... moon-tonk/
but, other good mechanical ones are:
mechanical pandaren dragonling - http://www.warcraftpets.com/wow-pets/me ... ragonling/
son of animus - http://www.warcraftpets.com/wow-pets/me ... of-animus/
clockwork gnome is annoyingly powerful - http://www.warcraftpets.com/wow-pets/me ... ork-gnome/
Getting all the pets listed here would not be a bad idea: http://www.warcraftpets.com/wow-pets/top-twenty/
However, regarding leveling, you can level pets really fast with this method:
I level 1 pet from 1-25 in about 1 hour.
Get 2 flying pets, i advise this guy: http://www.warcraftpets.com/wow-pets/fl ... hatchling/
, cos he has a self heal.
This guy is also pretty swell, due to immense damage: http://www.warcraftpets.com/wow-pets/fl ... ean-raven/
Once you have 2 flying pets at level 25, you go to the sha covered area, between the alliance and horde "cities" in the new expansion. Youll see loads of water pets around level 23-25, that you can battle. Chances are about 75% that on their first move they will not use an offensive ability, so you put out the pet you want to level first, make him attack or something, then swap to a flying pet and own them.
You can level faster if you have a sausage (25% boost), or that 50% boost thing.