I can’t figure out the best time or person, but I’m quite sure the best object to bring back is instructions for building the time machine.
I read in one of the Royal Diaries series, Lady of Ch’iao Kuo , where she mentions that she has had to go a long time in writing in her journal because she didn’t have access to supplies to making ink, so she couldn’t write.
That prospect seemed terrifying to me. Imagine being in a situation where you couldn’t write because you didn’t have ink.
So funnel in supplies to that period in Chinese history (AD 531) and other eras so people will have access to ink. The illiterate people can use it to paint pictures, or if they’re inclined, learn to read and write.
And since that funnel is open, also supply food and basic medical supplies as well to famine/poverty stricken periods.