The Leftovers is just a must watch.
Seconded! Fringe is also great, but I would watch The Leftovers first, it's much shorter.
If you do watch Fringe, you should actually skip the pilot and significant chunks of the first season, there are some mediocre episodes that have nothing to do with the main plot, it isn't until the end of season 1 that the show really kicks into high gear.
Start with 1.4 "The Arrival," and then skip to 1.14 "Ability," 1.15 "Inner Child," 1.17 "Bad Dreams," 1.19 "The Road Not Taken," and the season finale "There's More Than One of Everything."
For season 2, watch 2.1. "New Day in the Old Town, " 2.4 "Momentum Deferred," 2.6 "Earthling," 2.8 "August," 2.10 "Grey Matters," 2.15 "Jacksonville," and then... everything after that. Starting with Jacksonville, every episode is essential.