The 5970s and 6990s are definitely the most powerful, but its very hard to find a reasonably priced one right now. If the budget doesn't matter, there are a few selling on ebay. If your trying to budget, go for a high mhash/price cards, such as...
5850's which are great if you can find them for ~$150
I'm getting 350mhash/s out of both of mine
6950's are the next best and I just ordered two of the MSI 2gb 6950 reference models (http://www.newegg.com/Product/Product.aspx?Item=N82E16814127555&cm_re=6950-_-14-127-555-_-Product
) from newegg for $280 ($260 after rebates)
Should get 400+ mhash/s with unlocked shaders.
Regardless of what cards you choose, keep in mind that the current trend in difficulty has been about a 50% increase every 9-10 days... caveat emptor.