Well I'm currently using a Lumia 930 - is very good, WP8.1 is very smooth and the integration with the Surface is quite nice.
Also, my job (support for mobile phones, reviewing mobile phones, sometimes pre-release) does give me a lot of insight into this sector. I have used each of the 3 main OSs on my personal phones over the past 3-4 years (Android on a Galaxy S for 14months, WP7 on a L800 for 14 months, iOS on a 5/5s for 18 months, WP8.1 on a L930 for...4 days so far) so I know a lot from a usability perspective. And as we support every major (and most minor) mobile OS in my job I get exposed and have to use a variety of different devices each day and we get sent new ones and test ones for training/reviewing on the website, so although the following is personal opinion, it is all based on actual use of the devices.
Windows Phone is great, as said I'm using the latest and greatest in the 930, and having used a L800 way back when Nokia first jumped on the WP train, I have to say it has come on leaps and bounds. Aside from a 'lack' of apps compared to the other OSs, there is nothing I can think of that I'd want to change with it. And when I say lack of apps - most 'big' names have made apps for the OS, but it's all the smaller more indie developers still holding out until it becomes a more viable system.
The OS itself is stable, well built, clean and very Windows-y, and is still an 'exciting' OS, in that it still kinda feels like a work in progress so you get a lot of new features released and Nokia tend to update there apps very regularly.
The 930 is fantastic, you should go out and buy one right now.
That being said, I cannot knock Apple. My 5s is by far the best phone I'd ever had - perfect size for my hand (I find the 930 hard to use one handed), every app available, the camera is very good, and available accessories are great. From a usability standpoint, it is perfect. Apple do get a lot of hate just for being Apple, but having used the device, it really does do everything you could want.
Downsides are that it is still a bit restricted, but that looks to be changing quite a bit with iOS 8. My honest advise would be to not let an irrational hate for Apple blind you from what is actually a really good phone.
Android I'm still on the fence with. The Nexus devices are great for the money, and the OS itself is pretty much on par with anything now. However be careful what device you get, beyond a Nexus (and possibly a moto) manufacturers fill them with so much crap and bloatware you lose a lot of the great features and speed that comes with Vanilla Android. They are getting better, and can be very cheap, but again be aware as low end Android devices to be terrible.
They can also be a bit complicated if you're a non-techie, my dad's Xperia just decided last week it no longer wants to sync with the PC, no reason I can find, but took ages to reinstall everything and it still needs plugging/unplugging 4-5 times for the connection to hit. However, if you don't mind playing around and installing custom ROMS, skinning, theming - you can have so much fun just from personalisation!
In summation:
iPhone - Kill me for the cliche, but they do just work. Most of the time.
Android - Great for everyone really, but techie/tinkerer people really enjoy.
Windows Phone - Again great for everyone, smooth, fast and Nokia's hardware is really good.
BlackBerry? No. Just, no.
