Looking for suggestion on microcontroller
Beware that you are asking for opinions, which can be a reason for closing your question.

You stated both "clock frequency as high as possible" and "not that expensive", without quantifying either. This forces us answerers to guess what you actually want. Better state what you want to achieve (in terms of the timing of the waveform to be generated).

That said, I can suggest three routes (out of the many more that are possible):


go for an Arduino. complete hardware, including bootloader. lots of community support. Downside: the development environment is targeted (also) at non-techies, so you are somewaht shielded from the details.
go for Microchip PICs. Get a bare chip (18F4520 just to mention one), a breadboard, and a programmer (pickit2 or pickit3). You will be forced much more into the details than with the Arduino.
go for 32-bit, my suggestion would be the Cortex M0 LPC1114. I prefer C++, but C or asm is equally viable. You'll need a serial interface to connect to th chip and download a program (I use lpc21isp as tool on the PC).