need help! Opinion on best quality for a 15-20K budget

Hi Heather...have you seen diamond colors in person? I would recommend looking in person to determine your personal preferences with regards to color and size...that way you can decide how to best use your budget in a way that works for you. There is no 'perfect ratio' of diamond size to finger size...I would just get what you find appealing.

Ditto Slg. Do you have a Hearts on Fire Dealer near you. Leave your wallet at home (trust me) but make an appointment and have them walk you though the importance of cut... but also tell them you are interested in I and J color stones and SI clarity stones. See what they pull for you. Examine the rings in all kinds of light (walk around the store and away from the jewelers lights) and then compare the HoF to 'traditional' cut stones. Have them explain the difference between an AGS certificate and a GIA certificate to you as well.

Then come on here and we'll help you get a diamond as well cut as Hearts on Fire (they may have the worlds most perfectly cut diamonds... and no one EXCEEDS them, but there are some that can match them) without that insane mark up.

Also while you are there try on setting and decide:
1) four prongs or six
2) shank width

And then you'll be set and minimally educated so that you can find the ring of your dreams.

I totally agree with slg and gypsy. You need to go out and try some stuff on and get a basic education before you should consider buying a diamond online. What you *think* you like may be different from what you *actually* like.

Strongly suggest you to ask to be shown different quality diamonds so you can decide what grade especially color and clarity you like. You have a nice budget so look around, don't rush into anything. Happy shopping

My only caution is that the lighting in jewelry stores does not usually allow you to get a good idea of color differences. I'd recommend at least looking at this:

http://www.goodoldgold.com/4Cs/Color/

Most here go below D-F color to get a larger stone. G-H is at the high end of near colorless and quite white, and I-J will save you even more if you can handle just a little tint. The great thing is that ideal cut stones show little color face-up, so that allows one to go to the G-J range and gain some size!