If you're really expert and know what you're doing, for sure C is the most powerful programming language that allows you to think as a "machine". Assembly would be more performant but good luck not loosing your mind

For people suggesting any OOP-based language or GC-based language, you're going to waste some helpful CPU burst on checking pointers, and allocations. C is a nightmare for pointers but it can allow you to write very efficient program without too much efforts. Unfortunately you need to master pointers management and you need to carefully think about the instruction cycle (fetch, decode, execute), pipelines and much more to waste CPU burst as few as possible.