THAT is a loaded question. There are a total of five and two halves editions currently.
The most recent is Fifth Edition, and is rather popular at the moment.
The most common is 3.5, because it was the standard for nearly a decade, despite numerous mechanical flaws.
I like to compare the editions to Operating Systems for computers.
Chainmail is the early progenitor. It's like DOS.
DnD1: is Windows 1.0. It's ancient, but it started the trend.
DnD2: Is windows 95. It's what gave us the basic layout we're all familiar with.
Advanced DnD2: is like Windows 98: It plugged a lot of the holes in the previous version, added new functionalities, and made the system easier to use. It had its own flaws, but we kind of gloss over them out of respect for the many advances it made pushing the industry forward.
DnD3.0 is like Windows ME. It was there. We don't talk about it. And for damned good reason.
DnD3.5 is like Windows XP. It was the standard for a long time, it's what many MANY people were first introduced to, and it had the most support, despite its flaws and bugs, of any edition.
DnD4th edition is like Windows Vista. An attempt to take the niche successes of the previous edition and apply them to a broader audience while fixing the mechanical issues in the background. It fails at all of those goals and is often derided as an inferior product.
DnD5th Edition is like Windows 7, the company's apology to its customers for the last edition. A sincere effort produced by skilled professionals to create a stable product which provides exactly what the customer desires.
What edition gets played is entirely based upon the tastes and familiarity of the group in question. It was like pulling teeth to get my group to upgrade from 3.5 to 5th, and now they're doubting their own ability to go back to previous editions after enjoying it so much.