Unbord Val'kyr - Pandaren Water Spirit - Chrominius (basically, you launch all the delayed attacks from the first two pets, then Chrominius will lower their defense, all the burst goes off same turn, then Chrom drops his huge damage attack to finish)
There are other combos but they will become more apparent as you level pets and fight trainers.
Other really useful pets:
Flying: Gilnean Raven/Crow (ultra-high dps) Moths! Emporer Moth is easy to catch and level, Yellow Moth has the best power of all moths and is generally considered the top pick (great pick, fast, good movesets)
Aquatic: Pandaren Water Spirit (above combo) Emporer Crab (someone with Shell Shield and Renewing Mists, great tank) Eternal Strider (anything with Pump basically)
Mech: Clockwork Gnome (fast, has turret ability for great combos with lightning storm weather) Darkmoon Zeppelin (good dps, decoy ability) Mechanical Pandaren Dragonling (good dps, dragon ability on a mech body, decoy ability) Iron Starlette (anything with roll out ability) Darkmoon Tonk (charge ability) Sunreaver Micro-Sentry (lightning storm weather, has magic and mech moves, and his own dot to activate the lightning storm)
Magic: don't really use too many magic pets, but good ones I know: Legs Willy Viscidus Globule
Dragon: Emerald Proto-Whelp (P/P breed has huge healing and dps) Sprite Darter (another good dragon filler) Nexus Whelpling Celestial Dragon (magic moves on dragon body)
Humanoid: Kun-Lai Runt (P/P build has amazing dps, critter killer) Anubisath Idol (most versatile pet in game, amazing tank) Flayer Youngling
Elemental: Lil Ragnaros (pay to win baby) Lava Crab (f2p ragnaros)
Undead: Scourged Whelpling (dragon moves on undead body) Unborn Val'kyr (ascension is one of the best moves in the game, needed for combo above) Restless Shadeling Fossilized Hatchling
Critter: Stinker (bleet is best aoe healing ability in the game) critters are kinda whatever...don't really use too much, lots to pick from if you want, Rabbits are good I hear
Beast: Alpine Foxling Kit (anything with dazzling dance really) Chrominius (beast/magic moves with good debuff, part of combo above)