Budget? That's really the big question, because everything about the shooting depends on the exact lighting situations. Sometimes a simple 430EX2 will be enough, sometimes you'd need more.
If your budget is unlimited, it's easy - just go get a 600EX-RT.
If you don't want to spend $600+ on one Speedlite, then you can either go with a 430EX2 or 580EX2, or you could look at 3rd party E-TTL-compatible speedlites.
I've become a fan of Yongnuo, and they make the YN-565EX
that looks very similar in specs (and appearance) to the 580EX2, except it's only around $150. I believe that Sunpak, Vivitar, etc. also make E-TTL-compatible flashes for cheaper than what Canon Speedlites cost. In the long run, third party flashes probably aren't as well-made or reliable as Canon's, but in the short-term they'll probably do a comparable job, and it's a smaller investment to find out if E-TTL flashes will help your style of photography.