I used XE.com
XE.com when I had to change some currency to import an aircraft and at the time they were convincingly the best deal ( and I had an HSBC dollar account at the time which was the next best bet for large sums).
This was a while ago and you always need to find the best deal on the day that you decide to take the plunge.
Obviously the dollar will collapse the day after you do you deal but c'est la vie!
Cheers