Lost was at least a good show throughout its entire run (despite a dip in quality in the third season) and is honestly probably one of the best shows in TV history, despite its flaws. Few other shows have managed to continue building as extensive a mythology as Lost did without eventually collapsing under the weight of their own ambition. (Much as I love Battlestar Galactica, I have to admit that it definitely started to do this towards the end of its run, much more so than Lost did. The X-Files, another show I am quite fond of, and which quite clearly influenced Lost, suffered this problem even worse).
I do have to wonder if Lost would've been anywhere near as effective without Michael Giacchino's score though. Pretty much everything that man touches turns to gold.