Since you mentioned you're planning to use it in low light conditions, I'd vote for 135 f2. I own one and it's a great portrait lens, not heavy, so you could carry it the whole day without the tripod/monopod. I've never owned 100 f2 but I've heard a lot of good stuff about it too. Although it's not weather sealed, it's much cheaper that 135 f2.
It may sound silly, but have you considered Canon 90 f2.8 TSE? Maybe some of its owners can comment on it based on their experience but I think it should add some creativity to your photography.
Good luck with your choice. I think all three lenses you mentioned are excellent!