There seems to be huge number of meta tags you can set. I'm guessing that some of them are more important than others.
What are the most important metatags to supply and why?
Do I even need meta tags? (looking at the stackoverflow homepage there are none)
I'll use my same answer from this question:
A few years back, meta tags were important to search engine optimization. However, they've been abused and are generally ignored by almost all search engines (including Google, Yahoo and Live search. Excuse me: Bing).
The most important tags for SEO that you can include in your (X)HTML are the <title> and <meta name="description"...> tags.
<title> should generally be what you'd want the search engine to name your page in it's listing.<meta name="description"...> can sometimes give the search engine a basic idea of how to describe your page when indexing it.However, using these two tags will not necessarily make a difference in increasing your site's visibility on a search engines listings. For more information on that aspect, Google has a nice section on SEO on their site.
Update May 2015: Still being a very popular answer, I recommend you look at @alden's answer as it is much more updated then my own (6 years old now)
Original answer follows:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
To tell the browser what the content type and encoding is
<meta NAME="ROBOTS" CONTENT="NOODP">
See http://www.mattcutts.com/blog/google-supports-meta-noodp-tag/ for why
<meta name="description"...>
Obvious
<meta name="keywords"...>
Google do not use this, but other search engines may
<meta HTTP-EQUIV="Expires" CONTENT="Tue, 20 Jun 1995 04:13:09 GMT">
If you know when you like this page to be expired from cache
The most important meta tag you should use is:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
adjusted to suit. This ensures that, if you ever need to transmit that HTML document via something other than a web server (e.g. working on it locally, sending as an attachment), the user-agent is aware of its mime-type and character set. Just make sure your server headers agree.
the description metatag will be used by google as description of your site. I think it's the most important.
I think that the verify-v1 meta is important. It makes possible to use Google Webmaster tool
GoogleBot ignores meta information only trusting the page content itself rather than content descriptors. So, the usefulness of certain tags may be limited.