I have several decks that I use, but my favorite at the moment would be my Fire Princess deck. This deck (similar to Rebecca's deck, but I thought of it before seeing it in the show) is based on summoning Fire Princess, protecting her, and increasing your life points so that Fire Princess will slowly chip away at the opponent. It's not as difficult as it sounds, and if there are three Fire Princesses on the field the duel will end very quickly.

If they would release more Dark Magician support I would like it more, but currently there is only so much. One of my decks in Nightmare Troubadoure is a Dark Magician deck, but that's because they have cards like Dark Magic Curtain, Magician Cross, Magician's Circle (which I got with the game, but not until then) and Dimension Magic (just to name a few) which makes this deck semi-godly, not unbeatable but really good.

I suppose of the three god cards, Slifer is my favorite. Slifer's attack can easily be and stay at 6000, and it destroys just about an 4-star monster that might be summoned. Obelisk's attack remain at 4000, and getting two monsters on the field for its effect seems like more trouble than it's worth. Winged Dragon of Ra is interesting but meh. You'd have to use like three Goblin Attack Forces or Zombra the Darks to make it stronger than Slifer can be (and any good Slifer deck would have Infinite Cards to use). Also, if you use the ability to transfer life points to attack points, a Drillago could easily end the duel. I don't own any of the them though.