Get whatever you can afford. That's really all there is to it. You should also match it up based on the quality of your other components, so you don't end up with a bottleneck. Here's what I'd recommend.
Low/Mid End Computer - GTX 760.
Mid/High End Computer - GTX 770.
High End Computer - GTX 780.
Super High End Computer - GTX 780 Ti.
I wouldn't recommend getting any of the Titans, just because they're very expensive. Don't bother with the 5/6 series graphics cards, the new 7 series GPUs are much better. I'd recommend EVGA as your first choice of manufacturer, but ASUS and others are great too. Do your research and good luck!
EDIT: Also, make sure you get a card with at least 2GB of VRAM. If you can afford to get more, you should look at getting a card with 3GB or 4GB of VRAM. This will allow you to handle much higher end textures among other things, which is very important now that true "next-gen" titles are coming out.
