They don't have a particular order, I just write them as they come to my mind.
Probably the most important module of Drupal, you are nobody without. Views adds a visual environment to generate queryes the data base and to return the result in the format we want, plus there are lots of modules that increase the capabilities of Views.
Adds a "datepicker" widget to use it on our date type fields, amongst other things. Very useful for content-type "events", "courses", etc. .. integrates seamlessly with Calendar.
This is another of the modules that do everything but, in my opinion, the most important feature is that it allows us to create different types of fields to have them available when displaying content. For example, we can create a dynamic field that includes a view, a block, or whatever!. Also allows you to create layouts.
Allows us to group different fields in on content-type in div, fieldset, etc.
This module adds fields to make references to other entity (node, user, etc).
This is one of many modules that allows you to add WYSIWYG editor to content-types.
This is the most popular module in Drupal that allows you add a images to WYSIWYG editor.
This module allows you to easily create forms that the output can be sent via email or stored in the database. It also allows you to download the stored data into xls or csv format.
Finally, lets not forget to mention: CCK, ImageCache, Pathauto, Token, Captcha, Ctools, and Ubercart (e-commerce cart that integrates the most popular methods of payment. i.e. Paypal, Authorize.net, etc).