Vilegar (Vileplume UD with Spiriomb AR and Gengar SF)
DialgaChomp (Dialga G with Garchomp C)
Luxchomp (Luxray GL and Garchomp C)
BlazeChomp (Blaziken FB with Garchomp C)
Sablelock (Sableye SF with Garchomp C and Honckrow G)
Steelix Prime
Gyarados SF
Scizor Prime
Donphan Prime
Kingdra Prime
Jumpluff
Arceus
For what it's worth, not all of these decks are awesome. A new set is coming out soon, so the point is kind of moot, but at least for the battle roads season, this is what the tier system looked like...
Tier 1:
LuxChomp
Sablelock
DialgaChomp
Tier 1.5
BlazeChomp
Vilegar
Tier 2:
Gyarados
Gengar/Vileplume
Steelix
T-Tar Prime
Donphan
I guess you could say Scizor was tier two as well, but I think it's a bit below that.
As far as getting started competitively...
- Read http://google.com/.com, and if you're old enough, read heytrainer.org. Those are the two best websites for competitive discussion on the internet. Every site is good in their own way and important to the "eco-system" of Pokemon, but those two are the best for competitive discussion. Also peep JWittz' videos on Youtube.
- After reading those sites, learning about the tier system, what the best decks are, how they work, etc. See what's being played in your area (Battle Roads are over, but Cities are coming up), and based on that information, as well as your playstyle, come up with the deck that you want to run. The meta will be shifting with the new set coming out, but paying attention to the general meta online, and the meta in your area will help you out a lot.
- Read read read, listen, listen, listen, play play play. Those are the keys to winning. Read everything that anyone better than you posts, and listen to everything they tell you (not saying you should do everything they tell you, but give it a shot), and then play the game. A lot. Test test test.