I'm just gonna list my favorite 3 themes and some good cards that go with them.
 
1) Removal/Chaos; This is my favorite kind of theme. It uses monsters like Kycoo, Bazoo, and D.D. Crazy beast (all must-haves), Soul absorbtion and skull Lair for spell and trap cards. Most people overlook Soul Absorbtion, but it can be incredible with Bazoo, Kycoo, and other mass-removal cards comboed with Fire Princess. Remove with Bazoo and Kycoo, gain with Soul Absorbtion, and do Direct Damage with Fire Princess. There are a lot of removal cards out there, so you still have a lot of organizing room with a relatively strict archtype.
 
2) Stall/Gravity Bind; These decks are really fun to play, and can have a wide variety of cards. Gravity Bind, Dark Door, Toll, Gravekeeper's Servant, and Sinister Serpent are all must-haves. Man-eater-bug, Old Vindictive Magician, etc. also go in well. Since opponents tend to amass a huge arsenal of powerful monsters on the field in their spare time, I like putting in D.D. Trainer, (1 star 2000 DEF) and using it with Metamorphasis to get 1000 Eyes Restrict; you can then suck up your opponent's stockpiled powerhouses and attack with a 1-star! Also, Princess of Tsurugi and Just Deserts go well against a "stockpile" of cards. Other Direct Damagers, like Wave Motion Cannon, Tremendous Fire, Ookazi, and Gravekeeper's Curse) Finally, Legendary Ocean/Tornado Wall/ Gagagigo are good, stopping damage and having an ATK2050 monster with 3 stars (from the Legendary Ocean).
 
3) Warrior/Spellcaster or Warrior/Dragon; These decks can be extraordinarily tough with the right cards. The A. Forces is a must, Reinforcements of the Army is a good one, The Warrior Returning Alive, and Fusion Sword Murasame Blade are good for Magics. Dark Blade, Blindly Loyal Goblin, Warrior Dai Grepher, and Envoy of the Beggining are all good for warriors. If using Dragons, put in Dragonic Attack (for more Dragonpower), Kiryu (for Dark Blade), Kaiser Glider (good on it's own, can also fuel BLS - EOTB), Spear Dragon (best 4-star dragon), and Dragon's Rage (Fairy Meteor Crush time!). With Spellcasters, use Magical Marionette ( a great 6-star with insane effects), Skilled Dark Magician, Dark Magician, and Black Paladin. If you use Metamorphasis, use Dark Flare Knigt (offering Marionette/ Kaiser Glider) and Mirage Knight for some really awesome power.
      Either way, you rely on Warriors for lower-levels and the secondary type for higher-level bashing. This is my most powerful archtype.