vi: runs everywhere, installed by default on most linux distributions.
emacs: incredibly customizable, you can re-write the way your editor works. Some features that I find important: editing files remotely over SSH, running a shell inside the editor (so the same editing commands work in the shell), easy to write custom file finding functions for searching different parts of a codebase, git integration (a semi-GUI interface to git), and if another editor has a feature, someone has probably created a package that adds that functionality to emacs. Also has tetris and pong.
emacs: incredibly customizable, you can re-write the way your editor works. Some features that I find important: editing files remotely over SSH, running a shell inside the editor (so the same editing commands work in the shell), easy to write custom file finding functions for searching different parts of a codebase, git integration (a semi-GUI interface to git), and if another editor has a feature, someone has probably created a package that adds that functionality to emacs. Also has tetris and pong.