For Zekrom, I guess Naive or Hasty. Max speed is most important, and if you want both offensive stats to be usable, you'll have to sacrifice some of one of the defenses.
As for myself, I've beaten both games and absolutely loved the story. For my Black playthrough, I had a team of Emboar/Carracosta/Excadrill/Sigilyph/Sawsbuck/Cryogonal. A pretty solid team, though Cryogonal was underwhelming (I only kept it because it was a 1% encounter and had a Modest nature). For White, I spent days playing through the game because I raised 18 guys, including a Deino, Bisharp, and the Larvesta from the egg. It was extremely frustrating, but it was well worth it because I got to see how good they all were.
The one thing I dislike the most about the new game is breeding. You can no longer save and lock in the ability and nature of the egg, and the game really took advantage of that the other day. I was breeding for an Adamant Guts Timburr with Mach Punch and Drain Punch and got an Adamant one with 31/31/31/x/28/31. Naturally it had Sheer Force. I did eventually get a Guts one with 31/31/21/x/31/17, so it's okay I guess.
Wow, sorry for the extremely long post. I just learned an hour ago how to successfully post stuff on here because my screen reader seems to screw with me when it comes to typing stuff on web pages, and double posting breaks the rules.