1. Season 2. What a twist that was. In my opinion the 2 year twist was one of the best twists on television ever. Plus, it should be put in consideration that it truly changed the series as a whole.
2. Season 4. Very sudden, unexpected, really adrelaine pumping, intense. While the twist itself wasn't as "revolutionary" as season 2's, much of the shock value made up for it. ALTHOUGH... unfortunately, the cliffhanger didn't have much effect on season 5.
3. Season 5. No cliffhanger,only satisfaction. It was amazing.Such a great way to end a great show.
4. Season 1. Not very surprising, or anything, but it made the season complete.
5. Season 3. Bad. The problem is the writers when they did season 3, wanted something different from the ending. Then, when they wrote s4e1, they altered this idea. So... whatever.