Which one you choose depends on how much you are prepared to spend. All three are very capable GPU's and will play most games at 1920x happily with high detail/settings.
If it was me, and the budget stretched to it, I'd go for the very latest HD7950. But saving the difference (maybe put towards an SSD?) and getting a HD6970 is seriously worth consideration as well.
If I had an SSD, I'd go for HD7950. If I didn't, I'd definitely go for purchasing an SSD and a HD6970.