by far is a way of emphasising. e.g. "it's the best by far" (it's MUCH better than anything else) or "it's the worst by far (it's MUCH worse). So far means up until this point in time/until now. e.g. "So far I have only read half the book" or "So far I have visited two countries (but I want to visit more)". So far indicates that there is more of the action to do.