The meanings change depending on the context used. For example: "This is by far the best restaurant." Here "by far" is used to basically say that, compared to other restaurants, this one is the best. So here it is used sort of like a comparison among similar things. "You are by far the nicest person I've met." This is basically comparing "you" to other people. "so far" can also be used with the previous examples: "So far this is best restaurant" and "You are the nicest person I've met so far." Again it used as a comparison among things. But "so far" gives the feeling that okay, I still have to meet more people or go to other restaurants, but "so far" this is what I think of them. "By far" gives a sense of closure. "Bay far" and "so far" are mostly used colloquially. If you can provide the context these words are used in, it will be possible to give you a more precise meaning to them.