Reese Witherspoon's ring is classic and timeless just like she is. So, if you want to get a ring like Reese, she seems to have selected a mixed cut. So something between a step cut and a brilliant cut. It looks like to me it's a criss cut, but you could go for something like a radiant which is close, probably, as you can get. She has is simply set in a solitaire with diamonds down the side.
Classic, simple, elegant, Reese.