A lot of the strongest riders and most competant racers I know stick with Rival or 105. I generally buy Ultegra because I can afford it and I've never had a single problem with the series in 25 years of riding seriously. I'm sure I wouldn't be able to tell the difference between Ultegra and 105, but I figure that there's no reason to mess with success. I had DA on a bike once. Nice stuff but in no way worth the ~100% markup over Ultegra.