In no particular order
  1. When Jim finally asks Pam out on a date in the season 3 finale.
  2. In "The Conference" when Jim says that him and Michael used to work together but "now we're friends".
  3. The proposal. Obviously. Gas stations have never been more romantic.
  4. When Jim figures out that it's actually Michael's last day before he moves to Colorado and tells him that tomorrow he's going to take him out to work to tell him what a great boss he was.
  5. When Michael meets Holly and helps her work on the office chair. The Yoda voice gets me every time.
  6. When Kelly slaps Michael in Diversity Day
  7. Pretty much every part of the series finale but especially the part where you find out Michael has two phones to hold all the pictures of his kids. The scene where Michael returns after Jim says he can't be best man is also perfect.
  8. When Stanley proclaims his love for pretzel day.
  9. When Jim finds out that Pam is pregnant.
  10. When Ryan realizes he hooked up with Kelly the day before Valentine's Day. To me, February 13th is the real day to celebrate, solely for this reason.