I haven't been keeping up with the numbers, but based on my brief experience with the three OSes and pending products coming out, I'd say Windows Phone.
Using Windows Phone is a really cool experience. I think out of all the OSes out today, they have the most unique, clever approach to how a mobile OS should work. Microsoft also has a ton of hardware partners that can churn out a lot of devices on all the different carriers, so they have a pretty far reach. Once Nokia starts shipping their phone(s), their reach will be even more vast.
While I would like it to be WebOS, the realist in me says it will be Windows Phone 7. I think it will depend largely on whether or not either of them can attract that large segment of the market that is "first time smartphone buyers". I am convinced that this segment of the market, though, does not consist of people who care one whit about the OS. They assume a certain level of base functionality and choose their phones based on hardware. By those metrics, Microsoft is in a much better place than HP to attract their attention. That may change if HP licenses WebOS to other companies who have better ideas about hardware design.
I'll say Windows Phone 7, it's really cool and fluid and since they alliance with Nokia, their future is guaranteed.