I am willing to buy a fully licensed C compiler for 12f/16f and 18f 's.
I need a advice which one is better affordable and works best ( code optimization) ?
Why not just use the free version until you are more comfortable with it. I currently have a project with a 4*20 LCD, a 24LC1025 EEPROM, a DS1307 RTC and 3 one wire DS1820s. It's a temperature logger using a 16F886 and all the code fits within the limitation of the free version.
Another voice for BoostC, but it does not, I think, use the extended instruction set for the 18F series. How much does optimization matter ( how do you know )?
I think an important question here is, which ones are best documented, have the most example script, and are generally easy to get up and running with a given PIC. Because what really matters is if you will be able to use it. Unless your a pro anway, what you can figure out how to do with it matters more than what its theoretically capable of.
I guess another aspect is which gives supports with extensive library functions .
I agree CSS has a long list of library functions for most of the perpherials in a PIC microcontrollers.