1. Clean mark-up and standards compliant
This feature alone should be reason enough to use Drupal, it's not only great best practice in general, but search engines can easily crawl sites with clean mark-up. There are web standards for a reason, and search engineers are no strangers to these standards, websites that are compliant are more likely to stand the test of time and be looked upon favorably by crawlers.
2. Control over URL structure and path auto
This is what makes Drupal a stand-out from all of the other open-source CMS available. With relatively little work, the web developer/designer can enable clean-URLs and choose the URL structure for all of the site's content. Yes, it's so easy that a designer can do it. This, along with the contributed path auto module can automate the creation of search engine-friendly URLs. Path auto also gives you the option to write your own URL, so if you don't want the URL to match the title, then all do you is uncheck a box and enter what you want.
3. Control over page titles and meta tags
Add these two contributed modules and BAM! the SEO specialist has all of the necessary tools for making optimized pages. The page title module allows you to customize the <title> tag or, if you leave the field blank, it autofills with the title of the blog post or page. This is a win-win situation, it works for lazy people who want an appropriate title tag by automatically creating a title tag from the title. Also, it works for people who want to write their own title tag. The meta tags module allows you to create a custom meta description and add meta keywords.
4. It's easy to add a blog to a Drupal site
So you've finished developing an html site and now the client hears that this "blogging" can be good for business, so you spend 20 hours installing and theming a wordpress blog on the subdirectory mysite.com/blog. Not with Drupal. You finish developing and theming a great Drupal website with all of the appropriate SEO modules, then the client calls and says "I would really like to blog for my business." Ten minutes later you call back, "Done!" Yep, it's that easy, you don't even have to install anything, the blog module comes as a part of the Drupal core. When you enable it, clean URLs with your preferred URL structure will carry over to the blog as well. All you really have to do is configure permissions so your client can add blog posts, and maybe add a WSYIWIG editor so you don't have to teach anyone html.
5. Navigation appears at the top of page source
In some content management systems, the navigation ends up at the bottom of the page source after the page content, this is not great for search engines. You want the navigation front and center in the page source, and with Drupal, if you use the menus, the navigation will show at the top of the source with the content directly below that. Drupal also has really clean xhtml in the menus, making it easier for crawlers to get all around your site and see everything you have to offer.
6. Automatically creates an XML sitemap for submission to Google webmaster tools
Add another contributed module, Sitemap, to have an xml sitemap automatically generated. You can then submit this to Google Webmaster Tools so the search engine can find pages that it may not have otherwise crawled. Don't forget to update the robots.txt file to keep bots out of the pages you don't want them to crawl or index.
7. Google Analytics
8. Google website optimizer
9. SEO checklist module, so you don't forget anything
Having trouble remembering all the steps for SEO in Drupal? We add this module as a reminder to add all of the modules and setup configurations for SEO best practices. If you are a developer or designer and build & maintain a lot of websites, this module can help you manage the SEO for all of your clients' sites. Install the module and use it to check off SEO-related modules you have installed or other things you have configured. It's also a great way to show clients what you did above and beyond standard web design practices.
10. A passionate community with contributed modules to make it better everyday
The final reason Drupal is the best CMS for SEO is because Drupal is community driven. There are thousands of people using Drupal and creating, maintaining and updating contributed modules. If, next week, there is some huge breakthrough in SEO I can guarantee someone will be developing a Drupal module to supplement this breakthrough. It's not just a huge community, but also a standards-driven community. The people developing modules and upgrading Drupal core are committed to making Drupal the ultimate CMS and refuse to sacrifice web standards, upkeeping Drupal's reputation for search engine-friendliness.