It depends how much you have in your budget for this... If you're looking for the best hashing card, then the r9 290x will give the greatest hash rate. Most people will run some 7950s or 7970s as they return excellent hash rates, but are also a little bit more reasonable in purchase cost. Me? -I run a bunch of ASUS r9 280s. Even though they are essentially rebranded 7900 series cards, I'm hoping they hold a better resale value when I decide to stop mining and sell the cards ..we'll see about that. Each of these cards give about 750 Kh/s with the right setup in the config file.

I guess I should also ask, what are you looking to mine? BTC or another altcoin?