The sentence, "It is by far the best," is correct and can be used in written English. You can use this phrase when you want to emphasize something is of the highest quality or better than all the other options. For example, "This restaurant has the best seafood in town; it is by far the best!".
I'll be the first to say that Best Buy is usually nothing short of great to it's employees, it is by far the best company I've ever worked for, but they really dropped the ball on this one from an employee standpoint.
Numerically, going for it is by far the best outcome.
"It is by far the best study in marine disease ever," says Raimondi.
"It is by far the best conference in the country," he added.
Adbusters is not the only radical magazine calling for the end of life as we know it, but it is by far the best-looking.
"It is, by far, the best-selling branded board game ever created," Donovan writes.
"And it's by far the best one".
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK