The ones that matter are title tag and meta description as it is these which are used on the SERPs (search engine results page). They do not contribute to ranking improvements but are important with CTR (click through rate)... In otherwords its what people see in the search results describing your website... Hence a well written one will entice people to click on it.. Leaving it out all together and G will randomly select text from your web page - more often and not its a terrible description like the footer disclaimer or something completely unrelated lol... So use them for sure and you'll improve your CTR...
The other one and is also important is the canonicalization tag... This basically tells G which version of your webpage should take link juice credit...
For example:
http://www.domain.com/page1 versus http:// domain.com/page1
are treated as seperate pages wrt link juice attribution... By using canonicalization you increase the value of that page wrt it ranking better on the SERPs...
BTW - If anyone tells you meta keywords are important - then for sure run a mile as they know nothing about SEO and most likely trying to pull a fast one... Meta keywords is a relic of the past and has no value...
