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.
I agree with Mike go Canon. I really wanted the 580EX but I believe its almost twice as much as a 430 which is what I have. However the next flash I get will be the 580.
The second part of Big Mikes response is mine also. I have and really like the Sigma EF-500 DG ST This model exactly ! Build is same as Canons but a full step more in power. Price was $179.00, which you can't beat. Not a real fan of 3rd party anything, but this unit out-performs the Canon units
