By far, Windows Phone.
The Start screen's (I am calling it that) tiles are contrasting and easy on the eyes to quickly find things when you pull your phone instead of being distracted by graphic elements that add clutter. Live tiles ensure that you do not need to put additional time and effort to drive down to the app for pertinent information (a crux of now, some apps are not timely in live information). These tiles can be sized of importance - even more viable, given how the larger tiles can easily be accessed much quicker on a dime press - not worrying about zeroing in your fingers or thumb.
It is simple that there are two parts of the launcher aspect, the Start screen, and the "all apps" screen with a search icon (hey where have we seen this before?). There is also an upfront search icon on the phones themselves for just finding things.
App customization - that is not as important. I do not expect to do as much as I can versus a device with a much larger screen. What I do expect my phone to do, is being a VERY quick information portal and a VERY quick short communication platform (calls, being a given since it's a phone). I would say that I definitely appreciate it when financial apps now allowed mobile deposits.
The other nice thing upon initially using my HTC 8X, was the quickness of setup - initially and tying of my contacts from my previous phone and also additionally integrate online accounts. The built in mail program is very much viable for quick email receives and sends.
A limitation - IE on Windows Phone is limited to 6 tabs, and the only history would be relying on the hardware back button (which would include other previously accessed screens). There is other browsers, I have not tried them all, but so far out of the box, this is what is to be expected.
Another, would be as of now, a lack of an on phone file browser to traverse through files apps use to an overall user stored files (one would have to go to the apps themselves to access files). This is a double edge sword in terms of security and I see why it is sandboxed as such, but this is a concern with some users. Most apparent is the ConnectMe remote desktop app, anything I transfer over in a remote session, I simply cannot plug my phone in via USB cable and have access to those pulled files (because the seen storage via USB is segregated from the app's storage). This would be a deal breaker for many.
The modern right to left scrolling for additional tabs of some apps are very nice - it doesn't tie the phone's limited screen size and density with constant UI elements - and one has to play around to realize this - of which that is where intuitive comes about (because one can already know to do this just by sliding left to right on the Start screen and All apps screens).
Another confusion at the moment, is that I am much used to the Modern IE on Windows 8 with the Surface Pro - which I find myself at times using IE on my HTC 8X as such. If Microsoft is going to unify some aspects, they have other areas to address alongside their efforts.
The on screen keyboard (this is with relation to the physical screen size) is much nicer on my 8X versus my friend's iPhone 4. I find I have an easier, less stressing time texting on my phone and the 8X has a nicer grip. The displayed auto corrections is viewed in a horizontally presented listing that pares down as you type versus a single bubble on iOS.
I will definitely consider my next smartphone to be of the Windows Phone calibur and would recommend it as a good smartphone to start with in ease of use.