Tough call. Might go back to about 10-15kbce, just to see what was actually around then. Then again, hard to turn down a chance to co-exist with Jesus just so I could give everyone the real story first hand. If I felt selfish I could go back to about 1976-1980 to give my younger self a list of things to do and not do. Just thought of another: 50bce so I could find a way to save all the works in the Library of Alexandria before they all get destroyed.
Of course, there are some caveats. Can I take anything back with me, can I bring anything forward with me, will I be immune to diseases of the time/place, will I innately be able to communicate across language barriers, is this a 'can affect history' or 'can't affect history' timeline, etc.