Radeon HD 5830s, 5850s and 5870s are the ones for best cost-to-return ratios for any cards but the major issue is availability. They are pretty much sold out in every place you look or the prices have been jacked up. 3 5850s would be good if you can find them or the 6950 would work too. 6950 will be easier to find and they won't be outrageously over-priced. They won't get the same mining power as the 5800 series but will come close.
Actually the 6950's are on sale right now I can get them for about $230 each and looking up the benchmarks they seem pretty decent, I might pick up 2 of those. The 6970's are like closer to $400 each and while they have a bit more mining power they also only have like 3-10 more fps than the 6950 I find it hard to justify that...I also heard you can flash the bios to make it a 6970 but I have no idea how that works.