1) The Incident--Amazing, questions answered, Jacob/Darkman, the reveal of the bigger picture.
2) Exodus(season 1...but I don't think that's the right title.

)--Walt kidnapped, Sawyer shot, the hatch opening, the first of many stunners.
3) There's No Place Like Home--the island moving, Jin's 'death', Sun, the boat, Penny/Des reunion
4) Through the Looking Glass--Too much Jack crap or else it'd be number 2. Flipped the whole show on it's axis, the first of 2 game changing season finales, We have to go back!!!!
5) Live Together, Die Alone--Not that special, if I remember right the episode before this one where Ana Lucia and Libby get shot was far superior.