If you have no collection at all, I highly recommend getting HGSS packs first. Collector, Communication, Switch, Copycat, Fisherman, etc. Also, the primes from HGSS are all useful. Donphan prime goes without saying, Ampharos Prime fits in with basic yellow to make it slightly more powerful, Blissy Prime also fits in basic yellow. Gatr Prime is useful with Kyurem. Typhlosion Prime goes in tyram. Meganium prime is.... well, not that great but oh well.
Once you have a decent amount of HS packs, i'd choose what deck you want to build and go from there.
If you want a magnezone, machamp, mew, gengar, or yanmega variant, go for TM.
If you want to build stage two decks, go for UL (Rare Candy)
If you want to build a deck around the dragons, go for primarily BLW.
Which deck do you intend to build?