Lost would be my pick if I were limited to watching just a single television program. The downside of experiencing Lost is that everything else will likely fall short in comparison afterward. It’s arguably the very best TV series ever made, or at least among the top tier. Each installment delivers an incredible journey and features numerous memorable characters alongside captivating mysteries. Some viewers dislike the final episode because they didn't grasp its meaning; close attention to detail is essential throughout. I can’t think of any other show or film that I would suggest with as much enthusiasm, and it remains unmatched in my view—it really is something special.

Source on TMDB