Definitely the rarest backpack is War Backpack . In practice it is unobtainable (the only exception is to buy it from the current owner). Unfortunately TibiaWars, which got this backpack as a representing them item doesn't organise contests so there is no ability to win it in any way. What is more - War Backpack is only a decoration, it has no slots.
Quite rare, wanted because of achievement (and expensive at once) is Old and Used Backpack , which you can loot from rare Orc in Orc Fortress once per ~two weeks.
Another rare backpack is Ghost Backpack - according to TibiaWiki.com.br you can loot it from The Pale Worm.
Rare chance to drop has also Winged Backpack (Urmahlullu The Weakened), Book Backpack (Faceless Bane), Wolf Backpack (were-bosses) or Feedbag (Glooth Fairy).