In what order should I watch Durarara?
If you want to watch it from the beginning. Watch it according to this order.


Durarara Episode 1-12
Special 1: Heaven's Vengeance
Episode 13-24
Special 2: World at Peace
Durarara x2 Shou 1-4
x2 Shou Special: Watashi no Kokoro wa Nabe Moyou
Shou 5-12
Durarara x2 Ten EP-1
x2 Ten Special: Onoroke Chakapoko
x2 Ten - Remaining
Durarara x2 Ketsu


Sources:

List of Durarara Episodes on Wikipedia

List of Durarara x2 Episodes on Wikipedia

MyAnimeList

TheTVDB