Probably the most popular tournament deck today in the United States would be the Haymaker variant decks. The term was originally coined by Brian Brokaw (please correct me if I'm wrong) and Haymaker variants can usually be classified as depending on Big Basic Pokemons. The other telltale sign of a Haymaker is usually the heavy use of Energy removal Trainers.
I believe the current most popular design involves 4 Hitmonchan, 4 Scythers, 4 Electabuzzes, 4 Energy Removals, and 4 Super Energy Removals. Of course there are tons of variations, but that is probably the most popular configuration. Other Big Basics that also appear in Haymaker decks are Fossil Magmars and Promo Mewtwos.
Gligar is also an extremely good haymaker. 10 damage the first with a chance of poison. With 20 damage the next turn, it's a very good haymaker pokemon.
So why is this type of deck so effective? In order to effective use one and also to combat one, you'll need to know what makes it so strong. The list is actually quite long, but I'll try to go over the most important factors.
NO CHEAP KILLS. Most of the Haymaker Pokemons have 70 HP. If you're playing against one, you'll never get a lucky first round KO. 70 HP Pokemons actually take some time to knock out.
LOW ENERGY, FAST ATTACKS. Most of them require no more then 3 Energy Cards, but some are also very effective with 1 or 2 Energy, like Hitmonchan, Electabuz, and Fossil Magmar.
COORDINATING POKEMON TYPES. Electabuz works well with Fossil Magmar, as Lightning is powerful against Magmar's weakness. Scyther works well with Electabuz, as Scyther is powerful against Electabuz's weakness. Electabuz is powerful against popular Raindance decks.
HIGH DAMAGE THEME. Haymaker decks don't depend on Pokemon Powers nor do they depend on evolutions. Cards like Aerodactyl and Muk have no effect on Haymaker decks. They only depend on raw offensive power.
HIGH TRAINER COUNT. By far (in my opinion), this is the most important reason this deck is so effective. Twelve Pokemon and a lower Energy count allows for a massive amount of Trainer space. The more the Trainers, the more powerful the deck (not always the case). This reason allows for the following reasons:
SUPER ENERGY REMOVAL. Perhaps one of the most powerful Trainers, this card can completely devastate the opponent. In addition to Energy Removal, 4 of each of these Trainers can remove 12 opponent Energy Cards. There's a very good reason this Trainer is banned in Japan.
CARD DRAWING POWER. Since the objective of this deck is to win and win FAST, the Trainers usually involve multiple Professor Oaks and Bills. Card advantage will get you the cards you need whenever you need them.
Keep in mind these abilities of Haymaker decks when you're building or playing one. Try to follow the winning formula and you'll have a successful game.
In my opinion, Haymakers are extremely boring to play with or against. Super Energy Removal greatly detracts from the game, and there's no real fun to it. Surprisingly, I've won almost as many games stalling as I have from knock outs, all thanks to SER.
So remember, if you're playing against a friend, don't always use this deck. But if you're in a big tournament with good prizes, your objective moves from the fun side to lean over to the do whatever it takes to win side. Haymakers may be boring, but they win you games.