Hi, immortalLoser. Welcome to the TCG, as well as PokeBeach!
Looking at the current theme decks, Dark Hammer seems like the best one. It has the best Trainers out of any Theme Deck, as well as some solid Pokemon, such as Landorus, Pangoro, Hariyama, Machamp, and Hitmonlee. Landorus in particular is a pretty competitive card. If you're playing against other people with Theme decks, it won't matter too much which Theme Deck you choose, as they're all pretty evenly matched (unless one has weakness to the other, or course).
For other tips when just starting to play the game, check out
this guide. That should include all the information you need to know when starting out the game. For even more helpful information, check the sticky threads at the top of this forum.
I hope this was helpful to you!