Well, White Kyurem is by far the most effective competitively, because he can use his Special Attack stat more effectively, but I prefer Black Kyurem, because I think Zekrom's design mixes better with Kyurem's. White Kyurem completely destroys the awesomeness that is Reshiram.
Both Pokémon are unstoppable in-game, though. Freeze Shock and Ice Burn aren't that bad against the AI, since a fused Kyurem has monster defenses and can take hits very well.
I hate how people diss the basic Kyurem. It's a damn crippled dragon, of course it's not gonna be that strong. It's supposed to have trash moves. Smogon's profile of standard Kyurem is rather trollish.