The War of the Worlds

It is the mother of all alien invasion stories, and still, in its way, the most compellingly realistic. The biological truth Wells has given us would slow down an alien encounters on Star Trek or Farscape, where intrepid adventurers rarely worry about local languages, much less breathing the local air. The aliens have invaded because they have a fondness for human blood, sucked from living beings (how they discovered they had a taste for us is unclear, but it makes dramatic theater, anyway). If you haven't read Wells, you need to; Wells created a landmark -- he is a thoughtful social commentator, a pioneer of what makes SF intellectually appealing, and a damned fine storyteller, too.