I would recommend thinking very carefully about buying theme decks, and taking any gaming cues from the decks themselves. The decks generally only contain 1 or 2 good cards that are actually helpful, and in very low quantities. The theme decks also don't usually have a centered strategy that makes any sense.
If you're on a limited budget, I would say go for the HGSS trainer kit, like donavannj said, without a shadow of a doubt. Its inexpensiveness makes it a great way to get in the game, and the fact it comes in a half deck structure means that it's possible to experiment with cards with another player. Also a bonus pack is never a bad thing. Cards like the 2 pokemon collectors in the trainer kit itself are highly valuable in competitive play, so their average $3 price point coupled with the $4 pack adds up to a total of $10, plus the additional cards equals a virtually unrivaled value in terms of introduction play as well as competitive. Also it comes with a Pikachu coin. Either a chrome gold on the first release, or disco silver on the subsequent releases. Awesome.
I would also recommend that you locate a play! pokemon league near you, and get some deck building advice from one of the judges there, practice, have fun, and get some league promos and neat prizes. Leagues are free, you get prizes even if you lose, and great places to trade.
Ideally, I would say go for 2 HGSS kits if you have the $20 budget to afford them, since you then have some extra cards to experiment with, but if you can only get 1, then just get 1.
Personally I avoid buying straight packs, and always purchase blister packs or tins to ensure that even if the venture is a bust I have at least 1 playable card, but it can get pricey since the value of tins is pretty flat.
As for the actual question of what the best decks in the format are, I personally use a heavy fire deck based around Reshiram, but I'm only able to run it this season because I had some cards stocked up from a while ago, and even still I had to spend a considerable chunk of money and initiate a lot of trades to get cards to make my deck smooth. Other decks can get pretty pricey as well.
If you're looking to play competitively and get the money to get some of these cards, you might also consider selling some older cards from card games like Yugioh and Magic, or even old Pokemon cards if you played/play them don't use them or just have a heft of spare cards that you can get rid of. Companies like Troll and Toad buy these cards at gaming conventions, so if there's one upcoming near you that is also an excellent time to trade in junk for hard cash. The more you trade in, the more you get. Some places might even have cash bonuses for trading in Stacks of well over 1,000 cards, excessive amounts of rares, promotional cards, and singles.
I spent a decent amount of money on Pokemon cards overall over the past 2 years, and by trading in a a majority of the commons(about 400), selling a stack of unwanted rares, about 70 reverse holofoils, and selling some of my more valuable cards as singles, I made $170 cash off of it. It's not guaranteed that you'll make a huge fortune, but the guarantee of getting cash for trading in stuff you don't need is always satisfying. You can even find vendors selling singles for cheap, pick them up, and trade them for stuff you need.($9 Mint Machamp Prime FTW)
I think I might've gotten off topic, but I hope I helped.
