All of them, but 2000 is the most underrated (though not the biggest) one. The cultural polarization between Gore's smarty-pants liberalism and Bush's folksy conservatism is a good starting point for today's political divide. Look at the 1996-2000 county swing map (in relation to PV change) for reference:
Gore improved in virtually every major urban area and the black belt, as well as most of California, while losing significant ground in most of rural white America. The swing map has a lot of similarities to how one would expect the parties to do today, with a few exceptions. His improvement in Kansas and Tennessee for example seems to be a home state factor, Florida has trended right in relation to the nation despite Gore's improvements there, and almost all of Texas swung hard right, probably just because of Dubya. The most pronounced rightward swings outside Texas happened in Northern New England and upstate NY, Appalachia and around the Ohio river, the parts of the south located west of the Mississippi (i.e. less black people than the rest of the south), and a swathe of heavily rural counties stretching from rural midwest, through the Dakotas, through the rockies before stopping at the Cascades. Basically, most rural white areas apart from Kansas swung HARD to the GOP, even adjusting for the change in national PV. Also, notice Gore's improvement in the suburbs. Nobody was thinking of the burbs as Democratic just yet (because they weren't), but heavily suburban counties were already swinging to the Dems.
Even in Texas Dallas county and Harris county begins to trend D . In Georgia the suburban areas begin to trend D and VA NoVa trends D