Personally, I'd say the Garchomp theme decks (of you can get a hold of 2 of 'em) are amazing. It's a fairly consistent deck, and can stand up to a lot of stuff.
I think Mach Strike is currently very popular mainly because of Garchomp and Lucario combo which result in 200 massive damage. You can find out the strategy of this deck and other decks at here.
Tropical Take Down is also a good Theme Deck to use as a base, and in Expanded you can use Battle Compressor to discard enough energy to power the attack.