vim

vim (VI improved) is an editor that works very well in a terminal environment. The application is quite powerful, and allows very fast navigation and edits in files with very few keystrokes (and hardly any crtl, alt, which makes it RSI friendly).

1. The .vimrc file in your home directory helps making using vim a lot easier and convenient. Tastes differ, but I can recommend mine.
2. vim can be used to compare files side-by-side, scrolling synchronously.
3. By default, vim creates backup and swap files, which can be a problem for large files.