What 'contrasting' views have you heard? To some extent it will depend on the microcontroller and the application. However C is available for almost all architectures (I hesitate to say all, but probably all that you will ever encounter); so on that point alone, learning C would give you the greatest coverage.
For all architectures, the availability of an assembler and a C compiler are pretty much a given. For 32-bit and most 16-bit architectures C++ will also be available. Notable exceptions I have encountered are Microchip's PIC24/dsPIC parts for which C++ is not supported by Microchip's own GNU based compiler (although 3rd party compilers may do so).
While there are C++ compilers for 8 bit microcontroller's C++ is not ubiquitous on such platforms, and often the compilers are subsets of the full language. For the types (or more specifically the size) of application for which 8-bit is usually employed, C++ may be useful but not to the extent that it is on much larger applications, so C is generally adequate.
There are lot of myths about C++ in embedded systems; while the language is larger than C and has constructs that may compromise the performance or capacity of your system, you only pay for what you use with C++. But of course if what you use is just the C subset, the C would be adequate in any case.
The point about C (and C++) is that it is a systems level language; it will run on your microprocessor with no additional support save a very simple runtime start-up to initialise the processor (and possibly external SDRAM), initialise static data, establish a stack, and in the case of C++ invoke static constructors. This is why along with target specific assembler, it is used to build operating systems and kernels - it needs no operating system or kernel itself to run.
One of the reasons I suggested that it may depend on the microcontroller is that if for example it is an ARM9 with a few Mb of external SDRAM, and at least say 4Mb Flash (also usually external - memory takes up a lot of die space), then you could run a 'heavyweight' OS on it such as Linux, WinCE, or Symbian, or even a large RTOS such as QNX or VxWorks. Then your choice of language (once you got the OS working), would be influenced by the OS, though for real-time applications C and C++ would still dominate, (or often Ada in military, avionics, and some transport applications).
For mid-size applications - a few hundred KBytes of code and data space - C# running on the .NET-Micro platform is possible; However I sat in a presentation of this at the Embedded Systems Show in the UK a few years ago, just after it was when it was launched; when I asked the question "but is it real-time", and was told, "no you need WinCE for that", there was a gasp and a groan from much of the audience, and some stopped wasting their time an left the presentation there and then (including me).
So I am still interested in the 'contrasting' opinions you have heard; because although it is possible to use other languages; the answer to your question:
What language is most used in the
industry for microcontoller
programming?
then the definitive answer is C; for the reasons I have given. For anyone who might choose to contest this assertion here are the statistics (note the different survey method after 2004 explained in the text). However just to add to the collection of alternatives, I once spent two years programming in Forth on embedded systems, and I know of people still using it, but it is a bit of a niche.