Which is the most critical topic of computer science especially programming to teach at college/university Level?

i think system progroming or  compiler construction is the most difficult

I am not an expert to comment on the computer progamms and applications but as a learner I will rather suggest to have a good foundation on the general knowledge in the operation part and functional experience will help if it is given early in every ones life.

The most critical topic in computer science:

- Compiler Construction

- Integration of computer Electronics

I think computer science is an umbrella term which encompasses four major areas of computing theory, algorithms, programming languages, and architecture. "computer Engineering" typically focuses specifically on computer hardware and software. It is an integration of computer science and electrical engineering.

Programming in various compie languages 

Computer programing is the most difficult topic.

What I think can be difficult to explain is how data from the programmes are stored in the PC,... basically the memory logic. It requires some time.

compiler construction and LISP language means SCHEME BY MIT

C is one of the foundations for modern information technology (IT) and computer science (CS). Many working principles of IT and CS, such as programming languages, computer architectures, operating systems, network communication, database, graphical user interface (GUI), graphics, image processing, parallel processing, multi-threads, real-time systems, device drivers, data acquisition, algorithms, numerical analysis, and computer game, are based on or reflected in the functionalities and features of C. The experience in C will help students understand the working principles of these important concepts in IT and CS. Therefore, C is required for the CS major in almost all universities