I also say start with "Little House in the Big Woods" and read the Laura books in order. Some of the titles are pretty similar, but if read in order they follow Laura as she gets older. "Farmer Boy" is a departure as it focuses on Almanzo's childhood, so it can be read at any point; I think I read it last. They are definitely more geared towards children, especially Big Woods; I think I read it in maybe second or third grade, with my class. But as Laura matures the books mature as well, and as others have said "The First Four Years" is more adult-oriented, though fairly tame compared to modern adult stuff certainly.
I've never read "On the Way Home," "West from Home," "Old Town in the Green Groves," or "A Little House Traveler"--is anyone familiar with them? Is there a reason why they aren't included in the "standard" boxed-set series, or am I the remiss one? (They're mentioned on a Wikipedia page for the series.)
For me, the standard series is: Big Woods, House-Prairie, Plum Creek, Silver Lake, Long Winter, Town-Prairie, Golden Years, and First Four Years, plus Farmer Boy.