Wintergrasp for me. I love the full-scale battle between the Horde and the Alliance, especially defending with all the rushing to recapture points and destroy tanks. I find it especially fun when a particularly buffed up tena gets ganged on. Although I prefer running around defending, I wouldn't mind manning a tank myself, especially the gunner position. But catapults are still my preference when it comes to vehicles.
I know this isn't a battleground but, I used to love, back in BC, the massive scale fights in Isles of Quel'danas, usually over a dispute between mining which would eventually escalate into a 120 man war all over the island. My guild used to camp outside Sunwell Plateau and disrupt raids. We did the same with BT but it wasn't as fun since the place was so damn huge.
Other than Wintergrasp, my other favorites would be WSG and IoC, maybe even Arathi Basin. WSG for it's classic capture the flag, even though I lost 90% of the time. And IoC, believe it or not we played a hell of a lot of it, because it reminds me of a more elaborate version of AV. Arathi Basin because, well, I like to walk around and kill. Less technical more PvP.