Up to now; as of this time — often used in comparative or superlative constructions (e.g. 'the best so far').
今までで一番おいしかったケーキは、母の作ったものだ。
The best cake I've had so far was the one my mother made.
今までで最も難しい仕事を任されたが、挑戦してみるつもりだ。
I've been given the most difficult job so far, but I intend to take on the challenge.
今までで見た中で一番きれいな景色だった。
It was the most beautiful scenery I've seen so far.