I chose the Hirukawa lady for this one.
I think that the mystery was more of a set-the-stage mystery than anything else, probably so Gosho Aoyama could introduce Sera.
1. Hirukawa-san was the only one who's name was actually mentioned in the manga, indicating that she was an important figure in the murder case.
2. If we were judging by the time in which Hirukawa-san came down from seeing the Teigo-san and when he fell to his death at the parking lot, we could easily assume that she was the only suspect who had any contact with the victim.
3. Hirukawa-san claimed to be "scared to come out of the elevator", which may mean that she actually had some sort of device set up in a discreet corner of the elevator and was planning to dispose it (for instance, she may have stuffed it down into the elevator shaft) or was setting up something else. Also, she may have pressed the numbers 6 and 2, so that it would appear that the elevator had stopped on both of those floors before continuing to head down, giving the impression that somebody else was in the building.
4. Hirukawa-san was the one who suggested they go out to buy a recording device in the first place.
5. Hirukawa-san would have a motive: the death of her parents.