Ganking (or world PVP, if you prefer) is the main thing that differentiates PVP servers from PVE servers. Sure, PVE and PVP servers have the same mobs, the same raids and the same faction grinds, but somehow leveling just isn't the same without Xxninjaxx cheapshotting you while you've almost killed that elite mob.
Some of the most fiercely contested areas are the places where Horde and Alliance come together for the first time. It's no surprise that the lowbie areas of Ashenvale and Hillsbrad Foothills are widely considered the unofficial centers of gankage. Lowbie orc Bob gets ganked and corpse camped repeatedly by a night elf in Ashenvale, so when he's 70, he comes back and ganks and corpse camps all the lowbie night elves in revenge. It's a vicious cycle that can be scary to people going through the level grind for the first time. The frantic world PVP seems to level off a bit during higher levels, but anyone who was in Zangarmarsh around two weeks after TBC release knows that it can flare up at any time.
Personally, I found the most vicious zone for ganking -- on both factions -- is Stranglethorn Vale. I think I've spent more time in the graveyard at STV than I ever did at Blackrock Mountain. Maybe it's the 15 levels the place covers, maybe it's the confluence of questgivers at Nesingwary's Camp, or maybe everyone's having Apocalypse Now flashbacks, but you're never safe in STV.
Where have you been ganked the most?