The absolute best Vim command of all times is "ciw", or its variants "ci(", "cip", ... You use this command when your cursor is in the middle of a word (or parenthetical expression or paragraph or ...) and you want to replace that word by a new one.

Maybe it's too well-known, but this command changed my life and I haven't found in the linked list of TILs.

I got that one from Drew Neil's book: Practical Vim

Sweet find.

If you want to change a single line without doing a visual selection, '==' does the trick.