Melon is right. Ago wasn't always very pleasant. The average life span of a human being in the wild is about 35 years. But if you were only going back for a visit, what's the problem with the fact that the past wasn't as wonderful as the idealists imagine?
Anyway, top of my list, I'd want to meet Mark Twain.
Or I'd want to go back to a time before the extinction of the passenger pigeon. It was recounted that the sky would be black for hours when they were migrating.
Or back to the American plains before the railroads, to see the millions of buffalo on their migration.