Dragon or Ice. I personally don't either myself, but ice wins against so many types, it's hard to turn down. Dragon has only two weaknesses, Ice and Dragon. If there was an Ice-Dragon mix, I have a feeling it would PWN.
Dragon and Fire. Dragon effects every type except steal (I hate that >_<) and fire has 4 advantages, lots of powerful moves with a decent 3 weaknesses.
Dragon, Water, and Steel are the most useful types. Dragons only having two weaknesses, steel having a ton of resistances, and water having good coverage and being overall badass.