I put Greatest Hits on this list because of a few things. Its not one of the greatest episodes ever but does something that many of the great episodes of Lost don't. Instead of action, instead of WTF moments, and instead of twists, Greatest Hits is a touching memoir of one Charlie Pace.
Going through the years of his life, Charlie remembers the greatest moments and writes them down for Aaron. The apparent inevitability of Charlie's death hits home as the flashbacks register more as memories of a life troubled and reborn. I've never been totally sold on Charlie's death since I did not think it was completely necessary but this episode always helps to think about.
I do think that this episode is a glimpse of what's to come in season 6. Its going to be heartbreaking and there will be many more to die before the end. There isn't much more I could say about this episode, just that it was one of the most touching. It ends with the never before seen meeting of Claire and Charlie which could not be more perfect.