Let's start off with Black Kyurem and Zekrom!
Zekrom:
Zekrom, one of Unova's finest legendaries. Like you mentioned, STAB bolt strike, which can be devastating. But compared to most Ubers, he is a lil' bit slow to the ones he is weak to, like Garchomp. And he doesn't really have any physical coverage moves, so I guess if your making a special set include Focus Blast. His 150 attack is just amazing, and having used him, the stuff he can do is quite beyond me to be honest. And that's it. Zekrom should not be underestimated, unless compared to Garchomp, or Horn Claws Excadrill.
Black Kyurem
Wow. Look at that attack stat. It's not amazing compared to Dexoys-A, but still. Anything with a 170 attack stat should be stuffed and placed on a fireplace. I take pity on PO for moving it to OU. But now, I'm starting to see why. There is the problem of no physical ice moves, but even Skarmory cannot take it's Outrage. But again, he is weak to Stealth Rock, which can cause the worst of mistakes ever. If you are playing PO, always carry an Espeon if you're using Kyurem-B in a OU team. You'll thank me later :)
Overall, I would say Black Kyurem takes the biscuit compared to Zekrom.
Reshiram
I hate to start my comparisons with a bad note, but Stealth Rock. Mhmm. But apart from that, this thing ROCKS! Oh, I get it....
Zekrom can use Earth Power to cover it's weaknesses against Rock. and Solar Beam for ground. So Groudon isn't really a threat for Reshiram if you have sunny day up! It can use nearly all special moves affectively, and Draco Meteor will DESTROY. Seriously. But overall, well rounded stats. A great Pokemon!
White Kyurem
B-A-W-S. At least one of the Kyurem family stayed in ubers! Jeez. But again, Stealth Rock!
Even more Special Attack than Reshiram, still not amazing compared to Dexoys-A but again, 170 based stats are just terrific. Blizzard kicks ass, Ice Beam does too, and I'm pretty sure this thing has moves he can use he doesn't even need. I don't really see the point of Psychic, but ah well. It's got Earth Power to cover Steel-Rock types, so watch out Arceus Rock and Groudon!
Better than Kyurem-B, but not perfect. Again, for Ubers it's speed isn't amazing.
Conclusion: The Kyurem Forms are better. Fact
