Icon sets for Android are a dime a dozen, so it takes a lot of talent and effort, and a bit of luck, to stand out. Generally, icon sets come in three flavors: monochrome, standard, and uniform.
Monochrome icon sets are, well, monochrome: one color. Icons in a standard set are tied together by some sort of thread (flatness being the latest trend), and uniform icons are all the same shape.
Uniform icons are incredibly difficult to do well, because, inevitably, there will be a few stray icons that aren't themed, and those stick out like a sore thumb amidst an icon set in which everything else looks, geometrically, the same. Designer Sam Hewitt came up with a rather spectacular workaround for the problem.
The Moka set is indeed uniform, and quite flat. It's a striking set, and works with nearly any custom launcher on the market (I use Nova Launcher and it works beautifully), but the real beauty is in Sam's workaround. Moka is a squared theme with significantly rounded corners. In a recent update, the Moka application, once installed, themes any icon not included in the standard set by placing a grey background behind unthemed icons, giving them the appearance of being themed, and leaving every last icon on your system with the Moka look. Sometimes the approach works well, and sometimes it doesn't (see the Clean Master icon in the screenshot), but the effect is not nearly as jarring as it would be if the icons weren't themed at all.
The fact that Hewitt's solved the stray icon problem is impressive enough, but what puts this over the edge as the best icon set is the developer himself, who keeps fans up to date on the Moka project via dedicated Google+, Facebook, and Twitter accounts. He's also established dedicated forms for each platform for users to submit requests for icons that he's not yet themed- and in my experience, both on Linux (incidentally, a Linux icon set is also available) and Android, the updates come quickly. In all, I submitted eight icon requests, and the icon sets were updated within 48 hours to include my requests. I wouldn't expect that kind of turnaround if the Moka project becomes much more popular, but it certainly speaks well for the developer's work ethic and dedication to this project.
Between the sheer beauty of the icons themselves, the ingenuity of the uniform icon workaround, availability for nearly every launcher, and the dedication of the developer, Moka easily gets my vote for best Android icon set.