Reunion, for a number of reasons:
1. One of the most conceptually original tv shows ever, with each episode spanning an entire year, and the entire season spanning over 2 decades to alltogether create a mystery unparalleled in its scope and details.
2. The sense of mystery and revelation was second only really to Lost.
3. The nature of the show created an unparalleled scope for character development, where you got to see so many different dimensions to the characters across the years that the show spanned.
4. The acting, writing, and general plot structure were top notch to boot.
...and it had to get cancelled right in the middle of its mystery.