Things Should Know First

What do think after knowing these. What is yar choice…

Windows Mobile

C++ or .NET
free distribution, just like normal applications
Microsoft have market, too – here
Windows Phone 7
.NET and Silverlight
through Marketplace

Android

Java
through Android Market ($25 one-time fees) or like normal applications
The platform is completely open source

iPhone(my favorite)

Objective-C or Java (Developing iPhone Applications using Java)
through iPhone Market
pay some fees ($99/year)
The Incredible App Store Hype
You need Mac (Mac OS) for development (thx to ctacke)

Java

J2ME or JavaFX

Symbian

Is open source
Has by far the largest market share (45% or so) of smartphones
Runs on cheaper hardware than Android and iPhone (means volumes will go up significantly faster)
Runs Python, Ruby, Java and Web Runtime Widgets
From Symbian^4 will have a complete Qt stack.

According to me and fixEdd

Symbian’s dead; Windows Mobile will die, but take a long time; Android will become the standard in the next few years; iPhone will remain trendy in coming years, but NOT take be the biggest player; Pre/WebOS will maintain a niche market, but not be wildly sucessfull, Blackberry will decline, but still be around forever. I’d probably put my time/money into Android or iPhone at this point.