I don't pay much attention to any of them and never had any problems. Keep them all short and simple.
Meta Title has the most important role among all metas, however Search Engine don't pay much attention on this one. But since there is no harm on having this one, it's okay to have this as this could help in terms of snippets. Meta tags are the one will be display once a query is done on S.E
None of them are important for organic rankings. Honestly, <meta> elements (it's only a "tag" when you refer to the opening tag for the element or closing tag for the element) are for the most part ignored by search engines from a ranking perspective. Every search engine in which you should care about ranking ignores <meta name="keywords"> elements because of their history of abuse by blackhat SEOs. No reputable search engine uses this HTML element in their ranking algorithm. Google totally ignores <meta name="description"> in their ranking algorithm. This HTML element IS important, but NOT from a ranking perspective. Where the <meta name="description"> shines and becomes important is when it is shown in the SERPs as the snippet of text for your organic listing. A good <meta name="description"> can lead to a high click-thru-rate. A bad <meta name="description"> can mean hardly anyone clicks on your listing even when you show high on page one of the SERPs. Of course, <title> is NOT a <meta> element... But it IS the only thing in the <head> of your HTML document whose value is EXTREMELY important from an SEO ranking perspective. In fact it is the most important on-page SEO factor there is IMO.
Google has other way to, but for most other search engine meta tags are very important. Its the second thing that i user see in google result. of course the first in Title
Meta tag is very important in SEO, If you do not have any meta tag, it is not possible to take your site forward in SERP. It is very important, In meta tag, you will mostly include the keywords.
Title tag is the MOST important; description tag is largely for the human reader (when the scan the websites that come up in the serps) whereas - I have read - the keywords tag is not important/not so important.
Title and description tag are more important tags than keywords tag. No harm to putting kw tag on the page/s.