Tiffany & Co- the brand itself will sweep your fiance off her feet and at the same time burn a hole in your pocket. It's high-end, luxurious and very pricey! good workmanship.
Love & Co has nice designs and less expensive.
For better quality diamonds but not luxury brand, Tomei's medium-tier Le Lumiere is a good option. These are premium cut and more shiny.

http://www.wedresearch.net/2013/08/finding...engagement.html