Top 5 Drupal Modules

1. CCK - of course!
2. Admin Menu - I've been using this since D5.x and it really changed my life.
3. Image Cache (Image API, File Field, Image Field and Image Cache) - Really the only way to add consistent image content.
4. FCKeditor (IMCE, Image Resize Filter, Better Formats and FCKeditor) - For the end user or client, why do they still get into trouble? 
5. Google Analytics

So, really there is 11 modules that I've listed and I install all of these for every site. FCKeditor and Image Cache either have the listed dependencies or get installed to make full use of the features.
I would have to say Date API would be right up there also but not something I use all the time. There is also User Protect, which is important to save your super-user from being edited. Why is that not in core?