It's whatever you are most comfortable in, and can move the quickest.
It's the stack where you don't have to re-learn how to do things you know are possible.
So with that in mind, it's probably not the latest and greatest technology (except if you happen to have been living and breathing it for the past few years). It's probably got some warts that you don't like but for which you are also very competent at overcoming.
If you are leading a team and need to decide that for them, try to find out which stack each member has gotten the most done in (in current or previous job), and average it across the team. It might be helpful to set expectation that this is not the most glorious technological project of their lives but that the goal is a rapid developement project with the goal to replace parts with something more robust once they get validated on the product/user side.