Google used to recommend putting it just before the
</body> tag, because the original method they provided for loading
ga.js was blocking. The newer async syntax, though, can safely be put in the head with minimal blockage, so the current recommendation is just before the
<head> will add a little latency; in the footer will reduce the number of pageviews recorded at some small margin. It's a tradeoff.
ga.js is heavily cached and present on a large percentage of sites across the web, so its often served from the cache, reducing latency to almost nil.
As a matter of personal preference, I like to include it in the
<head>, but its really a matter of preference.