Colour D is the whitest Diamond colour, the lower down the scale you go, the more yellow the Diamond becomes, which to most people isn't very pleasing.
When I got mine, the jeweller recommended colour D-F as being the best. You could possibly get away with G, but having looked at a number of different colour grades together, you can see a definite difference.
The less inclusions you have the clearer the Diamond will look. F is obviously Flawless, so there will be no marks within it. VVSI (very very slight inclusions) would be the next best after that.
But if you are looking at the best colour in 1 Carat with the fewest inclusions, then expect to pay over �6 or �7K.
I think it's probably best to get the best Diamond you can afford, not necessarily be too concerned about the Diamond size. In this case, quality is preferable to size.