One of my favorite openings is when Pam was pregnant and asked people to stop eating smelly food because it makes her sick. Dwight then deliberately eats an egg and Pam pukes. Then everyone else starts puking after seeing Pam puke. It was classic.
I always loved their dynamic. It got kind of stale in 5 and 6 but has picked up in 7 now that they actually have a real role for Pam outside of receptionist.
Even Jim and Dwight got boring but that picked up again. Its like the writers really took time to think back to what made the first 4 seasons so great.