I'd pick the year on which a particularly significant natural phenomenon occurred, obviously one that's possible to determine with reasonable accuracy.

Ideally one that's distant enough to have most of human history on its positive side and whose consequences weren't only local.

Unfortunately I can't think of any event that fits the profile, but I'm willing to entertain suggestions!

hi! not discouraging responses here, but FYI, there was actually a very lively discussion on this ages ago, so check it out for previous suggestions

• If you were to make an alternative to BC/AD, what point in history would you start with for year 1? What's the first human milestone that you can accurately date?

and a related thread here

• Do any historians use the Holocene Era calendar in their work? (current year = 12013 HE)

It's a little arbitrary, as all of these sort of things must be, but I would probably choose the first print run of the Gutenberg Bible in 1455 as the dividing moment that separates the modern world from that which came before.

This feeling is based heavily on having recently been rereading and thinking about Marshall McLuhan's work. If you caught me after rereading Alfred Crosby I would have told you "1492."