Which is the best programming language for windows that supports GUI, USB, RS232 & preferably parallel interfacing?
I think all of those functions are supported in Windows through the
Application Programming Interface and Microsoft's Software Design Kit (SDK). Last time I looked the SDK was oriented towards the C programming language, but you are free to use whatever you like just so long as the proper calling procedures are enforced.
I think for high-speed parallel interfaces, interrupt-driven service routines written in Assembly will provide the best performance. But who wants to write another USB or RS232 UART driver when the Windows API already provides this functionality? Canned drivers for many parallel interfaces, such as so-called "Centronics" parallel printer ports are also available.
If you have custom-designed parallel interface hardware, you will probably want to write your own driver in C that interfaces to an interrupt service routine written in assembly. Well, that's how I approached the problem many Windows versions ago. Maybe Java Beans is the methodology
du jur at this point in the Windows saga...