Everyone has their own list. What are your top 10 episodes? And why is your #1 episode #1? Counting two-parters as one episode. My list:
1. Live Together, Die Alone
2. The Constant
3. Ab Aeterno
4. The End
5. Flashes Before Your Eyes
7. Through the Looking Glass
8. Deus Ex Machina
10. The Man Behind the Curtain
Live Together, Die Alone, one of the most dramatic, character-driven, yet eventful episodes was - to me - Lost in all its perfection. You had new mysteries, answers to old mysteries (including one of the biggest of the show at the time), you had almost Desmond's entire background, you had new Dharma stuff, you had Others kidnapping Losties, and you had a major plot device blown up. What the episode included:
-Desmond big time - Desmond is released from prison; Widmore (first time introduced in the show) tries to bribe him; Desmond has a heartfelt conversation with Penny with Jack running in the background; Desmond meets Libby and gets her boat; Desmond shipwrecks on the island, and is rescued by the mysterious Inman; Desmond pushes the button for 3 years and learns some more about Dharma; Desmond kills Inman, and weeks later is about to commit suicide only to be saved by the sound of John Locke banging on the hatch door because Boone just died.
-One of the biggest mysteries of the entire series (at that time) was answered in this episode - Desmond is the one who crashed the plane by not pushing the button!!! NO ONE saw that coming.
-The Hatch, the centerpiece of the entire season (and much of the first season as well), is blown up, by Desmond making the ultimate sacrifice.
-Henry Gale, the timid and sleezy man they had prisoner, is revealed to be the LEADER of the Others!
-Crazy background crossovers - we discover that Desmond's boat (which ultimately survived to the finale) was given to him by Libby, who is later killed in his very own hatch. The man who rescues Desmond and lives with him for 3 yeas in the hatch is revealed to be the same man who served with Kate's father and taught Sayid how to torture.
-After spending most of season 2 hating on Charlie, Claire forgives him and they share a passionate kiss.
-First time we see the now infamous statue
-The Hurley bird makes his first appearance
-Michael's treacherous actions are uncovered and revealed
-Cliffhangers galore - our 3 heroes Jack, Kate, and Sawyer are captured by the Others - what's their fate? And even bigger, did Locke, Eko, and Desmond survive the Hatch explosion?
-The first time any of our heroes actually escapes the island (Michael and Walt)
-And this one's a big one - it's the first time that we see any present day off-island events (Penny's men locating the island from the hatch explosion), acknowledging for the first time that the island was neither hell, purgatory, nor existing some time in the distant past - all among the very top theories at the time.
ALL of that was in LTDA - the best of the best of Lost.