There is no such thing as minimum meta tags (unless of course I got your question wrong). As far as I am aware no meta tag is required and the ones you add are the ones for your specific needs.
Consider the following document:
<!DOCTYPE HTML>
<html>
<head>
<title>Some Title</title>
</head>
<body>
</body>
</html>
You can validate it and not get any warning whatsoever. The validator just reminds you that the default encoding is missing. This is not even a warning, just an information.
The working draft has this to say about meta tags:
The meta element represents various kinds of metadata that cannot be expressed using the title, base, link, style, and script elements.
And it goes on:
4.2.5.1 Standard metadata names
application-name, author, description, generator, keywords
Further it mentions some additional tags concerning a page’s status, contextual representation and character encoding.
Although none of these ar explicitly required by the standard, there are in fact best practices, especially concerning search engine optimization (SEO). This has nothing to do with the HTML standard but with web (search) crawlers.
You can get some good advice which meta tags matter (for Google) at the Webmaster Tools meta tag support page