The single most useful Emacs feature

The single most useful Emacs feature
stackoverflow.com

Hi-lock lets you specify regexes to highlight anywhere in a file; it's like domain-specific font-lock.

It's astoundingly useful in code review. For instance, given a giant blob of J2EE web handler code, I can eyeball one handler, recognize the annotation mapping the method to URL syntax, and then punch in a hi-lock regex to light up similar annotations everywhere in the file, then glance through the whole file looking for color patterns. I can read any of those functions and recognize the code that checks "am I an admin", hi-lock it, and then quickly spot any function mapped to a URL that doesn't check admin credentials.

That's just one tiny use case. I used hi-lock so much I ported it into our web-based code review tool. I converted one person to Emacs just to get that feature (my office would be thrilled if someone could point out the vim package that does the same thing).

CUA column editing (CTR-SPC arrow-arrow-&c type-type-&c) is probably my #2.

Late edit:

Holy hell how do you forget magit! Magit is a near-complete UI for git; it figures out what git repo you're working from, and then gives you up-to-date status on the contents of your checkout, spots untracked files, allows you to visually stage your commits, gives you single-key revert and a browsable repo history.

Magit is one of the rare Emacs extensions that not only improves Emacs but also the external service it integrates with. I am actually a better git user (or, git is just better) when I've got magit running.