What meta tags should not be missing in the header of a good site? [duplicate]

Title

It's not exactly a meta tag but after the content of the page is the best thing in terms of SEO. Do not use more than 70 characters. It is unnecessary to specify the name of your site here, use only a text that is strongly related to the content of the page. Capriche here!

<title>O título vem aqui</title>

Meta description

The description of the page. Useful for SEO. Maximum of 160 characters.

<meta name="description" content="descrição aqui">

Meta robots

It tells search engines certain procedures regarding the page. Values:

• index : the page should be indexed (default)
• noindex : page should not be indexed
• follow : page links should be followed (default)
• nofollow : Page links should not be followed
• none : same as noindex, nofollow
• noarchive : searcher should not save cached page
• noodp : the search engine should not use the description of certain link directories (eg dmoz.org)

• nosnippet : causes Google to only display the link to your page at the time of search, no longer displaying the description, etc. (not recommended)

NOTE: default values do not need to be specified, ie it is unnecessary to use index, follow

NOTE: I particularly recommend using noarchive to prevent Google from saving your page, because if it saves a particular page and then you regret it, it's a bit difficult to remove. Some people like Google to cache just in case the site goes out of time temporarily, so people can still see the content of the page.

Meta googlebot

Same as robots , but is specific to Googlebot.

<meta name="googlebot" content="noindex">

Meta keywords

Keywords related to the content of the page. Google no longer uses this meta tag, but other search engines (like Bing, Baidu) can use it.

<meta name="keywords" content="palavra1, palavra 2, outra aqui">

Meta author

The name of the person who created the page content. It does not help in SEO, but Facebook for example shows the author specified in that meta tag when you share a link.

<meta name="author" content="João Silva">

Meta google

Tell Google that your page should not be translated by Google Translate. This causes Google to stop displaying the "Translate this page" link next to your site link on the search page.

<meta name="google" content="notranslate">

The most basic and non-existent meta tags are:

<meta name="description" content="Descrição sobre o conteúdoda página..." />
<meta name="keywords" content="palavras, chaves, sobre, seu, conteudo" />

Following these, would come the Robots tags, which configure and guide the search engines indexing thefts, but these are no longer being used as much as google mechanisms already do the job automatically:

<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
//Não indexar e não seguir os links

<META NAME="ROBOTS" CONTENT="INDEX, NOFOLLOW">
//Indexar, mas não seguir os links

<META NAME="ROBOTS" CONTENT="NOINDEX, FOLLOW">
//Não indexar, mas seguir os links

<META NAME="ROBOTS" CONTENT="INDEX, FOLLOW">
//Indexar e seguir os links

If you need to, you also have some meta tags that control navigation such as:

<meta http-equiv="refresh" content="30">
//Tempo para atualizar a pagina

<meta http-equiv="refresh" content="x_seconds; url=http://www.yourhost.com/pagetosendto.html">
//Redirecionar a pagina após x segundos

And last but not least, the meta tags that social networks use to capture page information when the link is shared:

//Facebook
<meta property="og:title" content="..."/>
<meta property="og:description" content="..."/>
<meta property="og:image" content="http://sua_url.com/foto-facebook.jpg"/>

//Twitter
<meta name="twitter:title" content="...">
<meta name="twitter:description" content="...">
<meta name="twitter:image" content="http://sua_url.com/foto-twitter.jpg">

I hope you have helped.

If you have any questions or want to see more meta tags this post of acquire is quite complete on the subject.