I haven't bought too many theme decks, but I found that the Forest Guardian deck from EX Hidden Legends worked really well after a bit of modification. I just got rid of a few bad pokemon that didn't lead anywhere (surskit, dark celebi), cut a few energy, replaced a few trainers, and I added in a bulbasaur line with venusaur ex and the pop2 promo venusaur. Belossom helped heal, the deck moved relatively quickly... It wasn't tournament worthy or anything, but it was nice for just playing with friends.