Must have for human readable urls instead of urls like /node/1
You will want to control each pages title tags and meta description for search results listing and SEO.
You will want data on who is visiting you website and their interactions. This modules allows you to track customize which roles you track.
Simple XML sitemap
You will need this for SEO and Google Webmasters tools et al.
If you plan to let users sign up and sign in this is a must have module. Spam bots can easily detected Drupal’s login url and form so protect yourself before you have a lot of annoying accounts to delete.