Worst launches so far (EU) were LK and WoD.
Naxx 25 was literally unplayable for at least 1 full month after release at peak server times due to lag. We kept wiping to the easiest raid bosses ever due to our screens stopping for up to 1 min...
WoD... we all know how it went. About 2 days of being stuck in or around garrisons.
Others were seamless or almost seamless. Haven't been there at the vanilla launch tho'.