What programming language to use for microcontrollers

Depends on the microcontroller and the task. Most microcotnrollers these days support C, so unless there is a specific requirement that cannot be solved in C (for example, if it is not fast or small enough), then assembly.

If the microcontroller does not have a C compiler or has other high level languages, then obviously you would want to evaluate them to see what the best option is for your project. The same goes for the decision of which microcontroller to use. It depends on what you need: unit cost, performance, power consumption, available development tools/compilers/languages/documentation, available I/O pines etc etc.

As for what microcontroller I'd use - if I wanted to do somehting simple as inexpensively as possible and don't require high performance or many I/O pins, I would use a PIC [1] microcontroller, as they are well known (therefore are reliable, have tools and documentation), extremely cheap and low power.

If I need something more powerful, need more I/O pins, need 32bit integer support (or generally can deal witht he fact that they cost maybe 15x the amount of a PIC), I would use a Parallax Propeller [2] microcontroller. Its a great architecture, is easy to program (in its own interpreted high level language, Spin, in assembly, in C and in Forth - possibly other languages too now), has good documentation, sample code and an active community and is quite powerful. Its funky 8 core interrupt-free architecture is interesting too.

If I needed something even more powerful than a microcontroller, I'd use an ARM Cortex A8 [3] based processor.

If you need support for peripherals or otherwise want a more complete package than a microcontroller or processor, I'd use either a Propeller-based Chameleon [4] or a Cortex A8 based BeagleBoard [5].

[1] PIC - http://en.wikipedia.org/wiki/PIC_microcontroller

[2] Parallax Propeller - http://www.parallax.com/propeller/

[3] ARM Cortex A8 - http://www.arm.com/products/processors/cortex-a/cortex-a8.ph...

[4] Chameleon - two models, 16bit PIC & Propeller: http://www.xgamestation.com/view_product.php?id=52 and 8bit AVR & Propeller: http://www.xgamestation.com/view_product.php?id=51

[5] BeagleBoard - http://beagleboard.org/