The strongest of the starters is the Arcanine/Venasaur (in my opinion).
I bought the Milotic one last week (I just re-started) and I already have it a completely different deck and none of my friends can beat it. I doubt it's tournament worthy, though.
But the booster packs i was buying were pretty much random.
I'd go for Rising Rivals, though. It has the most cards that would benefit the deck.
And if you buy the packs with 2 Rising Rivals and another pack, get either Legends Awakened, Secret Wonders, or DP Stormfront.
The main cards that would benefit you out of the sets are:
Legends Awakened -
Regice
Deoxys
Mesprit
Uxie
Azelf
Froslass
Kyogre
Secret Wonders -
Mew
Suicuine
Blastoise (if you want to get rid of Swampert, Mudkip, Marshtomp).
Gardevoir
Stormfront -
Empoleon (again, if you want to get rid of Swampert).
Gengar
Gyarados
Roserade