The best, top of the line, option would probably be the Canon 580EX II. It's rather expensive though. The next Canon model in the line up is the 430EX. It's slightly less powerful but still pretty powerful. The controls are better on the 580 but that may not be a big deal. The biggest difference is that the 580 can act as a wireless 'master'. It can remotely control other 580 or 430 flashes. The 430 can only act as a 'slave' and not a master. If you only ever plan to use one flash, then this is not an issue.
Another option would be the Sigma 500 Super. It's almost as powerful as the 580 but around the price of the 430. It will work with the camera's TTL (through the lens) metering, just like the Canon models. I've heard that the build quality isn't as good as the Canons though. Actually, the 580EX II, is weather sealed, which is a nice plus.
There are plenty of cheaper options but I would recommend one of the Canon units.