Using joomla since day 1 I often re-use a lot of joomla components over and over. There are a lot of components that give Joomla the power to be the best CMS on market today.
Here are the most useful components I used in the years for building sites. Of course the list is not exhaustive. I do not claim to have used all components and also I will not include any commercial components in this list. Commercial components will make another list .

The list is not in a specific order, the components are all good and mainly from different areas – so it would be hard to compare them to each other..