iOS : B : Closed off, just like the people that buy the devices. That said, it does few things wrong and is very accessible.
Android : A- : Open and closed, hot and cold. Maturing fast in both the OS and sales.
WP7 : C- : Trying to hard to separate itself while lacking many key features, such as multitasking and C&P.
WM6/6.5 : C : Easier grade for being a product of a simpler time, still terribly flawed.
Blackberry OS6 : D : When you get down to it a prettier version of 5 with only a handful of upgrades.
WebOS : C- : Capable, but not very user friendly and never pushed to its potential.
Symbian^3 : B- : A valiant attempt, but too little too late.
Symbian 5th : F : Has not aged well. At all.
Maemo 5 : B : Does whatever you want it too, providing you know ho to make it do it.
What's in store for the future? As far the the iPhone is concerned, nothing. 4 is still new and has all but tapped its potential as far as new features are concerned, a lot of that due to the nature of the OS and the AT&T network. One of either of the Android versions 2.3 and 3.0 (Gingerbread and Honeycomb) are supposed to completely retool the Operating system for the first time ever which, along with ever-evolving hardware, will lead to an even better OS. Unless it gets new, exciting hardware Blackberry can kiss its marketshare goodbye. Microsoft has good hardware, but even with its monster PR campaign too many people have been burned by Mobile 6 to try Phone 7. Nokia's Symbian would need a push like what MS is giving WP7 in order to succeed, which it probably won't get, at least in the US. WebOS could also succeed, but to do that it can't get by on user experience, it needs to show how powerful and flexible the OS can be, which would require a giant effort by HP. Maemo is turning into Meego, which is potentially exciting but unless Intel is truly committed to the project it will remain a fringe OS and not the Symbian replacement Nokia is hoping for.
In short iOS and Android are the future, and I predict one of the other big 4 (BB OS6, WP7, Web OS 2.0, Symbian^3) will emerge as a viable third contender while the other three hang on to a tiny slice of the market, with Web OS possibly dying or at least being removed from phones and put into printers and TV's and fridges and whatnot.
For now I can only recommend Android as iOS5 has little room for growth, WP7 has too many bugs, Blackberry continues its slide, Symbian gets ignored by the public, and WebOS gets ignored by HP.