Is it <!DOCTYPE html> ?
This is not really a meta tag. Technically it’s SGML – Standard Generalised Markup Language, which came before HTML. It goes at the top, just to say ‘this will be a HTML doc’, but if you take this line out, and just go straight into <html> Google won’t care to be honest. It’s a very minor accessibility issue, very minor SEO effect if any. I guess there’s an argument to say it’s a kind of meta tag
Would it be the meta tag that permits crawlers to index the site?
Yes! That’s one of a few possible answers.
It could look like this: <meta name=”robots” content=”index, follow”>
It could be Google tailored: <meta name=”googlebot” content=”noindex”>
But ‘robots’ is more common, targeting all engines together, ‘robots’ is recommended.
This tag is important because if you get it wrong it can kill the whole site.
There are a few other tags capable of this, for example: https://www.w3.org/TR/WCAG20-TECHS/H76.html
If you redirect all users away instantly, or with a few seconds delay, it’s basically killing the indexability of your page. Google and W3C both recommend not to use this type of redirect.
<meta name=”google-site-verification” content=”…” /> …This one is quite critical also; because if you verify, or someone else verifies on your site, someone has the ability to login to Google Search Console and tell Google your whole site has moved, ignore the whole site, etc. Quite critical.
Those are the critical technical meta tags, with the maximum danger if you get them wrong. Assuming you don’t have such ‘critical technical errors’, what is the next most important meta tag for SEO?
Could it be <meta name=”description” content=”…”> as this features in the snippet?
Most people would say <title>
Although technically not a meta tag in the same way as <meta> tags are, it’s generally still meta data because it’s in the <head> and in theory all info in Head is Meta.
There is also a true meta tag like <meta name=”DC.title” content=”Title goes here”> – although Google doesn’t talk about this one, it sometimes does use it to override <title> if it thinks <title> is spammed but the DC.title is a genuine title.
It also looks at backlinks – if everyone links to your site with the same anchor text Google will treat that text as a good title, and if the <title> you made is spammy Google will often fallback to the anchor text based title.
Similarly if your logo links to your homepage from every page on your site, the ALT text of the logo is a place Google finds worthy title text in case the <title> looks spammy or otherwise inadequate.