Diamond and Pearl Block:
Majestic Dawn
The only good card getting out of this set is probably Call Energy. So it's not advised to buy any packs from this set also because of how likely it'll be rotating out.
Legends Awakened
Uxie, Azelf, Mesprit, Regice, and a lot of other smaller cards came out during this set. You could get lucky and pull the coveted Uxie Lv.X but due to how many Lv.Xs are in this set it is not a smart choice to try and hope for it. Also because Uxie Lv.X's price has been dropping. And as before, it is most liekly rotating out. Because Uxie and Azelf are going to be staples for almost any deck you make you can just go to a league and try and trade for a few Uxie and pick up an Azelf as the league promo card.
Stormfront
One of the most influential sets we've had. If you don't mind about your cards "going bad" in about 6-8 months then you can buy this set. Comes with many popular archetype cards like Gyarados, Machamp, Gengar, and a few others that could make excellent decks. But once again, only try and buy singles of this set.
Platinum
This is where it starts getting tricky. Platinum comes with a bunch of staples for the SP player like Cyrus's Conspiracy, Energy Gain, Power Spray, PokeTurn, and many other Pokemon SP like Crobat G and Bronzong G. By far the best set to buy (if you're not paranoid about what the rotation is going to possibly be). Other mentionable cards besides the SP ones are Broken Time-Space, Pokemon Rescue, and to a certain extent Looker's Investigation.
Rising Rivals
Don't really know what to say about this set. I guess just a bunch of once metagame cards (except for Luxray GL Lv.X). Not really worth buying this set because you're just better off buying two Luxrays for the price of one box. SP Radar is also a staple for SP decks if you do happen to make an SP deck.
Supreme Victors
Interesting set to say the least. The hyped was overhyped and the unhyped was underhyped. Only useful things in this set were the normal SP forms and their Lv.Xs. The best probably being Garchomp C, Blaziken FB, and Staraptor FB and their Lv.Xs. Palmer's Contribution and VS Seeker are nice to have but you only really need to own about 2-3 of each at most.
Arceus
I guess the highlight of this set are Charizard, Spiritomb, Gengar Lv.X, and Expert Belt. Unless, of course, you plan on making an Arceus deck because this set provides one of each regular Arceus. You'll then also need Beginning Door and Ultimate Zone (also from the same set). Spiritomb and Expert Belt can be received from your local league (if they still have any left) while Charizard is being sold in stores as a 1-1-1 line with a pack.
LEGEND Block:
HeartGold and SouldSilver
HG/SS created what our format is today. It gave us back Double Colorless Energy which is most commonly seen in decks that include Garchomp C and its Lv.X. Jumpluff and Ninetales are both good holofoil rares but don't see as much play in today's game as what they used to or expected to. Pokemon Collector, Pokemon Communication, and Copycat are my pick as the top 3 Trainers from this set. Pokemon Collector is used in just about every deck while you can find Pokemon Communication in ALMOST any evolution based deck and sometimes SP decks. Copycat was reprinted in this set and so if you have any older ones that'll be good. In my opinion, the only useful prime from this set was Donphan and even he isn't used that often anymore.
Unleashed
Decent for its Rare Candy reprint, Tyranitar (Prime) and its line, Steelix (Prime), and Entei & Raikou LEGEND (most useful during last season's nationals). Cool set with some cool Pokemon, not much else.
Undaunted
Interesting set considering that people that it was pretty bad. Vileplume is good as well as Smeargle. Really, this set gave quite a few decks different techs to be able to use. It also gave birth to the nuisance named Umbreon. Only useful prime in the eyes of a competitor is Scizor (Prime). The rest of them don't fit into the metagame all that well at this point.
Triumphant
You could choose Triumphant and hope to get lucky by pulling the 40 dollar Gengar (Prime). But don't base you're decision just on that alone. Just about all of the primes that came from this set have a use, but unfortanetly you'll need multiple copies for them to work so that's where trading comes in. Machamp (Prime) is decent as its own deck with the Machamp from Stormfront. 150 is massive (even for a stage 2), and being able to use its Poke-Power and then use Seeker to pick it back up and heal makes it a nuisance. Speaking of Seeker, that's one of the most single influential cards that we have right now. Rescue Energy and Junk Arm are also decent so it would never hurt to have some of those on hand. Overall though, besides the primes and a few of the trainers, the set doesn't really seem all too appealing. Remember, you're not guaranteed all the cards from the set in just one box.
Call of Legends
DO NOT GET THIS SET. CALL OF LEGENDS IS GOOD FOR COLLECTORS ONLY. Yeah, it was a lot of reprint cards that COULD be useful for you like the Smeargle from Undaunted and the Ninetales from HG/SS, but there aren't too many cards from this set that'll help you get started. Very difficult to even make your money back from the box.
I'm not gonna tell you which one you should buy because that's really up to you depending on what you're looking for. Besides that, good luck getting back into the game. =]
