Jack and Sayid are the two most capable people on the Island, and they know it. Sayid provides the tactical and technical skills Jack lacks; Jack provides the moral compass Sayid sometimes loses. Their bond is forged in Season 1 and holds through every crisis. Sayid's sacrifice on the submarine -- running with the bomb to save the others -- is the soldier's version of what Jack does at the Source. Both men die protecting the people they love.

1. S1E2
   Pilot (Part 2)

   Sayid fixes the transceiver. Jack immediately trusts his skills.

2. S2E15
   One of Them

   Sayid interrogates "Henry Gale." Jack confronts the moral cost of what Sayid can do.

3. S6E11
   The Candidate

   Sayid grabs the bomb and runs. "There's no time." His final act saves the group.

Season 1: instant mutual respect. Seasons 2-3: Sayid handles what Jack can't stomach. Season 4: both leave the Island, both broken by it. Season 6: Sayid is "infected" but breaks free at the end to save his friends.