For all of these addons what you'll find in common is that they all work very well. They are exceptional in functionality and features. They are all designed well so that it is easy to use. Plus, they are all very well supported by the developers that created them, continually providing bug fixes and updates. If you want examples of the best of the best, here are five.
For all of these addon modules, 3 Red Kites have been using them for years, so yes, we're biased because we like these ones.
Our five favourite Expressionengine addons are, not in any particular order:
1. Pixel and Tonic's WYGWAM - making the difficult in-browser wysiwyg even better
2. Pixel and Tonic's Playa - making it easy to relate different entries
3. Low's Variables - easy snippets of information
4. Solspace's User - enhances membership abilities
5. Solspace's Freeform (or Freeform Pro) - greatly enhances the collecting of form information