Lost at 15
What separated Lost from the pack is that it had compelling characters, a great cast, and an enticing mystery that invited fans to solve it. Lost was one of the first shows to embrace AR storytelling, which has now become ubiquitous. Other networks, and even ABC itself, tried to recreate Lost's magic formula ... and have yet to find a way to recapture that lightning in a bottle.
But all good things come to an end, and that's the biggest problem some people have with Lost: the ending. It must have been impossible to craft a conclusion that would please everyone, but the Lost finale was unusually divisive. Some even claimed that it retroactively ruined the show for them. We wouldn't go that far. If the Lost finale had truly been that bad, we wouldn't still be talking about the show's beautifully convoluted mythology a decade and a half later.
LOST was monumentally epic (just read through some of the comments on this forum thread at MacRumous, classic). It had a mythology that captivated millions (and continues to do so to this day). It was ahead of its time, inspired countless sci-fi series, horror anthologies, films, and kicked Bad Robot Productions into override to produce sleeper hits such as, 10 Cloverfield Lane. It led Abrams to the directorial helm in Star Wars: The Force Awakens. Lastly, until HBO's Game of Thrones, LOST previously held the contrarian title of worse series finale ever.
LOST was and continues to be so important to me. I think I might be overdue for a re-watch. We have to go back...