I agree with the above post regarding the center stone. However, if you are referring to setting designers, I absolutely love my Ritani setting. You can see all of their designs at http://www.ritani.com.
def Tiffany. Its more reasonably priced than some of the others (cartier) and you know the stone is of supurb quality. It may be more expensive than purchasing from blue nile or other retailers, but you know you are getting quality and dont need to haggle, etc. Diamonds are an emotional decision and lack logic all together, so going for a better diamond and a good buying experience is in my opinion better than going for a "good deal."
No BRAND is best. I tend to prefer avoiding the brands and getting TripleIdeal cut stones from a vendor online like Good Old Gold (goodoldgold.com), and buying a setting I like, even if it's another vendor.
Based on my AWFUL experience with one particular online vendor starting with the letter W, I STRONGLY recommend you never patronize them.