Vim's modal editing is the best system for manipulating text.

As specified in the title, it is flexible as nowadays it contains a significant number of plugins and it is also very fast (if you know how to optimise it and use it wisely).

It is one of the earliest text editors that works fantastic on Unix/Linux based systems on the fly. Today, I still use vim on OSX for simple text editing. It works right in terminal, so it allows you to text edit files while SSH-ing into a remote machine.