During an emergency swimming ascent where should the regulator be?

[Torpedo]

Can some one tell me what is the best?

[John deep]

you're required to remove it when doing the open water course. It's part of the skill set required to pass. If you suffer a free flow, I'd be interested to see how fast you fill up with air with that reg still in your mouth. You are likely confusing "don't hold your breath", with don't remove the reg.

[Zeus]

I gather you're asking this about an out of air situation. That reg still stays in your mouth. As you ascend, the gas in your tank expands slightly as the ambient water pressure decreases. The first and second stage can take advantage of that and also the because of the lesser delivery demand because of that decrease. You'll get another breath out of it. It may be the difference between being bug eyed or not.

[Mako]

During an emergency ascent the regulator must be in your mouth!
The air in the second stage of your regulator and hose could expand and eventually give you some air to breath or the failure could return to normal.
But most important it will prevent you to inhale water!
Mako

[DiveDevil1966]

I am going with Mako on this one. The regulator should remain in your mouth during the "Controlled Emergency Swimming Ascent" in order to protect the airway as well as the possibility of a bit of air on the way up. However it depends a bit with which training agency the diver is learning to dive, as there are slight differences and variations in the training approach.
I think one of the most important issues here is that during the training the emphasis is also what to do when finally reaching the surface. That's where the diver (in most cases) has to orally inflate the BCD. Incident reports suggest that many divers having problems under water actually make it to the surface, and then fail to inflate there BCD.