I'd go for public_html first, "most likely suspect." Servers are generally configured to look for indexing in a particular order, like index.html, if not found, index.html, if not found . . . others, depends on what's in your configuration. So index.html should be fine - but a small hint, for links back to "home page," don't do
<a href="/index.html">Home</a>
do
<a href="/">Home</a>
This gives you the freedom to do things with "home" that may or may not involve changing the extension, without having to consider SEO issues.
