Windows for Mobile? Well, "best" is a term about quality, not deployment or acceptance. A lot of people believe (with reason) that Unix/Linux is the "best" operating system, and it owns the majority of the server market. Yet it's less than 1% of desktops -- it's a complete failure in that market, when taken by that measure.
If in your opinion Windows for Mobile is the "best" mobile OS, that's fine, opinions vary and everybody's got one.
But the chance of Windows Mobile getting serious market traction in the presence of iOS and Android is about the same as Linux "taking over the desktop" while Windows is around. Not holding my breath.
"This is the year of the Linux desktop" has become a standing joke among Linux fans. Not about quality, just percentage uptake.
"This is the year of Windows Mobile" has just about achieved similar status. And again, not about quality, just percentage uptake.
Indeed, best does not always equate to most popular.
I think Windows Mobile can be successful in the enterprise and the professional class (a la Surface). The consumer market is a different story however; I don’t see Microsoft ever making a consumer phone, so it will be up to OEM partners, such as Acer, to make the push there.