For a single card, it's the GTX 780 at $650. If you can't spend that much, look at the GTX 770 for $400. You could also look at GTX 760s in SLI for $500.
On multi-monitors, a single card is not going to allow the highest settings for most games. That's why I'm giving you Nvidia Surround options. AMD Crossfire suffers from frame pacing problems in multiple monitor/Eyefinity resolutions. What that means is that a pair of cards delivers gaming smoothness that is no better than a single card.