when I m analyzing keywords for my site, which is the best- long tail keywords or short tail keywords?

Thanks for your help !

competition is much harder for short tail keywords especially if your website does not provide unique contents. And for a new website sometimes it becomes almost impossible to compete in short tail keywords

you should define yourself what do you need traffic or rankings, if rankings then - long tail, if traffic - short tail

I suggest Long tail keywords. in these keywords contains multiple words of keywords like Organic SEO Services its a keyword contains combination of words (Organic, SEO, Services). Search engine provide the results on the base of combination of keywords.