It's hard answering this question, because "used most" implies the need for some kind of statistics to back it up. Instead I present you with some examples.
Objective-C is more or less the default language for iPhone and iPad applications. Flash is verboten, but I think Adobe AIR is allowed, and MonoTouch (.NET)
Java is commonly used for Android applications, although you can probably use any language with a JVM application, so technically it could be Scala, Rhino, Jython.
You can use Lua, Python and more on Android with a program made for running scripts, released by Google outside of Market I think.
Nokia Maego and Symbian favours C++ if I recall correctly but I wouldn't be surprised if you could use virtually anything provided it is runnable on the platform.
Anyhow it varies a lot, and depending on the OS vendor, you can often use a multitude of languages as long as they allow it.