I Can Hear The Heart Beating as One. Then Fakebook. Then every other record.
These are excellent places to start. Fakebook is my second favorite. I would almost say, though, for someone starting and not going chronologically, both And Then Nothing Turned Itself Out (my favorite) and Painful are diverse and give a good sense of Yo La Tengo's breadth.
Whatever you do, please don't start with Summer Sun. You might stop there.