The newest you can afford.
They are all good machines, every year.
But, the older they are, the more problems they usually have.
Because the older they are, the more likely they are to have high hours, have been abused, or are just worn out.
Or, if you find an old, low hours, factory looking garage queen, it can have issues just from sitting for so long.
But, if it runs good, looks good, no smoke or engine noise, looks taken care of, and if the seller is knowledgeable(knew how to take care of it), and if the price is right, buy any year you like.