Right now, there are 4 pretty good contenders for winning U.S. nationals this year.
Magneboar
Donchamp
Reshiboar
ZPS
Allow me to explain.
1) Magneboar- Out of all of these four decks, this deck has the highest chance of winning if you ask me. Magneboar is overall a pretty good deck. It is pretty consistent and can deal lots of damage since it is all thanks to Magnezone Prime. Its power proves very good drawpower, and its attack is also pretty good, virtually able to KO anything with almost relative ease. Emboar helps to makes it attack more efficient by attaching lots of fire energies around the field, and put in some more cards like RDL, Tyrogue, Cleffa, and Reshiram and you have a pretty good deck. I won't be surprised if this deck wins our nats.
2) Donchamp- Basically the runner up for most likely to win U.S. nationals. It is perhaps the best deck out there as of now that uses Donphan. Donphan can hit quick with Earthquake, and its body makes it pretty hard to KO for most Pokemon. If it is still around, it can even go for a good 90 damage with Heavy Impact. Then in the deck comes Machamp. Machamp Prime can hit very hard with Champ Buster, and it has the potential to OHKO anything if all of your bench has damage counters on them, and this achieved through Donphan's Earthquake. Being a fighting-type deck, Donchamp has type advantage over Magenboar because Magnezone is weak to fighting. Of course, there is still the chance that Magneboar can beat it back. Otherwise, with ZPS winning Canadian nats, this deck is expected to see play nats for sure, and there is a good chance that it can win.
3) Reshiboar- It is very much like Magneboar, only you use Reshiram for attacking instead of Magnezone. The strategy is very much the same, with Emboar attaching up the fire energies to make attacking quicker and more efficient, and then there's a hard hitter that benefits from that, Reshiram. Reshiram can hit for 120 with three energy on it, and then 2 :fire: energies must be discarded afterwards. Emboar helps to make Reshiram attack every turn with an attack like this, although it can use Outrage from time to time if it has a lot of damage. The only problem with Reshiboar compared to Magneboar is that Reshirboar has huge weakness to water, in which anything water is basically an autoloss because Emboar and Reshiram share a water weakness. Because of this, Magneboar is often the better choice, but Reshiboar is still a good deck to use, and there's a nice possibility that it could win, but not as much as Reshiboar and Donchamp.
4) ZPS- ZPS stands for Zekrom, Pachirisu, and Shaymin in case anyone is wondering, and those three cards is what makes the deck. Zekrom is the big heavy hitter, hitting for 120 for three energy, but it does 40 to itself afterward. Since it can't discard energies, this makes it have a nice energy advantage compared to Reshiram. Pachirisu and Shaymin help power up Zekrom, and the combination of the two can even cause Zekrom to pull off a KO on its first turn, making Zekrom are very good deck to use early game. The 40 damage backlash allows it to hit harder with Outrage, making it able to pull off KOs on most basics. Despite the great early game advantage, there are problems for Zekrom. One is that it has a bad matchup to Donphan. Donphan can easily OHKO a Zekrom after a Bolt Strike since Zekrom is weak to fighting. That's why people use Yanmega Prime to help out against the matchup, but sometimes Yanemega won't get very far. The other problem, which is perhaps the deck's biggest problem, is that it has a pretty bad late game. Zekroms can get pretty well damaged after a few Bolt Strikes, making them easy for a KO. Therefore, there's a good chance your opponent will catch up to the point of winning. Due to this, ZPS doesn't have a good chance of winning compared to the other three decks.
So in the end, these four decks will probably win U.S. nationals, with Magneboar being the most likely. However, there are other decks that could win as well. These decks include Yanmega varients, Samurott/Donphan, and Lostgar, although Lostgar seems to stand of the three decks just listed. Who knows what will win though. We'll just have to see.