1 x 2560*1080 screen. It's wide enough to side by side just about anything I need to, which is indispensable. If I didn't have a widescreen, I'd definitely want two monitors. That being said, I feel the need for more pixels...
For me personally the change is due to
1. No longer need a status/tracking window
2. O/S supports multiple Desktops which are Retina (high res) so using another screen downgraded image.
3. Resolution I use is high, MacBook Pro run at 2?xHD
4. MacBookPro notebooks are design with gestures which mean from a neck & hand position basis.. everything stays in one place and I just swipe fast between desktops. This seems more efficient than looking at another screen (despite crappier resolution).
5. Only time I use screen it’s for a Vm and the. It’s an effort and I can’t just pick up notebook and walk around(have to unplug hdmi, drag desktop to MacBook ideally)
Dell P2414H and the Newer P2417H is the most popular however, due to them being cheap. Having both HDMI and Displayport, also VGA Ports, and they can adjust up and down (so no monitor stand or telephone book as some use), and they can swivel 90 degrees (nifty for long spreadsheets.)
Also if you're running HDMI or Displayport, their picture quality is hard to beat, unless you're a graphics designer...
Then you should get the Dell UP2716D (Ultrasharp Professional Dreamcolor with 1 billion colours thats factory calibrated for perfect colours). For those of you that dont know. Most monitors support far less than 1 billion colours, and if you're into digital photography you need to match perfect colouring for print. As such your monitor usually have to be calibrated to nearly 1% accuracy across the frame. They slap on a colour calibrator that's effectively a camera attached to the front of your monitor to check for the picture colour accuracy and brightness.) Even a 10% difference which is normal in standard monitors, can be catastrophic in photography).
Apple Advises the usage of the UP2716D for photoshop work and as the main monitors for the Powermacs and Mac Minis.