Well maybe after today it will soon be the newly announced Jira OPS? :-)
In all seriousness, for me it has to be Jira simply because of the far reaching impact it has on our business. Many groups use it throughout the day to drive everything they do. We use it to measure our progress. We us it to release our products. We use it as part of our collaboration. I would say too that JSD is a close second for similar reasons.
