You use the expression by far when you are comparing something or someone with others of the same kind, in order to emphasize how great the difference is between them. For example, you can say that something is by far the best or the best by far to indicate that it is definitely the best.
[emphasis]
By far the most important issue for them is unemployment.
It was better by far to be clear-headed.
See full dictionary entry for far
by far in British English
by a considerable margin
See full dictionary entry for far
by far in American English
considerably; to a great degree; very much
: also far and away
See full dictionary entry for far
Examples of 'by far' in a sentence
by far
By far the most important skill at the bridge table is counting.
Times, Sunday Times (2017)
But most important, by far, is the number of times consumers wear a garment before throwing it out.
Smithsonian Mag (2017)
His own moments of brilliance were outnumbered by far too many basic errors.
Times, Sunday Times (2012)
Credit cards that are specifically designed for overseas use are by far the cheapest option.
Times, Sunday Times (2014)
Yet by far the most important of the three was the deficit.
Times, Sunday Times (2010)
That issue is by far the key in any size church.
Christianity Today (2000)
That was by far the most important thing.
The Sun (2011)
Instead he introduced a system of fees that were by far his most important legacy as a reformer of the public sector.
Times, Sunday Times (2010)
By far the biggest issue is lack of self-confidence.
Times, Sunday Times (2014)
This is now a third in six months, and by far the most important.
Times, Sunday Times (2012)
Of those, by far the most important is to refuse.
Times, Sunday Times (2016)
Indeed politically, it is by far the most important thing to happen this year.
Times, Sunday Times (2015)
By far the most important of the three is Fit.
Times, Sunday Times (2010)
The economy is by far the biggest issue in the campaign with two-thirds picking it in their top three.
The Sun (2010)
There are myriad links in the ivory smuggling chain, but by far the most important is the final one.
Times, Sunday Times (2014)
By far the biggest issue was PPI.
Times, Sunday Times (2011)
I think there were three factors, of which the last is by far the most important.
Times, Sunday Times (2015)
By far the least substantial of Olympic buildings; by far the most important.
Times, Sunday Times (2012)
And by far the most important element in that equation is energy: mainly emotional, partly physical.
Times, Sunday Times (2015)
The most important of these is the economy, by far the most important factor driving the Remain vote.
Times, Sunday Times (2016)
Chinese translation of 'by far'
by far
... 得多 ( ... de duō)
⇒ She was by far the best swimmer.她显然是最好的游泳者。 (Tā xiǎnrán shì zuì hǎo de yóuyǒngzhě.)