A real holodeck of course.
Though I would hardwire the security protocols - no way to disengage them at all - and make sure then if there are any problems with the power supply, the thing will shut off.
Also, after 8 hours of running, the thing will always shut off no matter what, and cannot be restarted for a full hour, and theres a crowbar to open the door if the controls fail, and there is a big OFF button to cut off the power supply once the holodeck is off.