For me it was Wintergrasp during wotlk. My first maxed 5 man team was a few months into wotlk and playing on a pve server(cause I didn't really know better)....nobody was ever flagged. So WG was my only shot to wpvp. And looking back now, I kick myself for how little I understood the shaman class then, but how well they performed boxed. Which is the completely opposite now.

My favorite part was flying into the room where you entered the raid( after horde had won WG) and just completely wiping half the raid waiting outside this instance. And then basicly camping the instance location not allowing them to enter.
Those were the days!