How to pour water from a bottle as fast as possible?

I can only give you a hand-waving explanation, but the fastest way to empty a bottle I have empirically found is to pour it upside down but rotate the bottle. The main problem is that the empty space above the water needs to be filled again with air which can not get through. If you rotate the bottle correctly, an air corridor inside the bottleneck opens and air can constantly flow back into the bottle, thus creating a nice constant stream of water around the side of the bottleneck and no "spluttering" behaviour.

To reflect and preserve a few useful additions that were made in the comment section:
Peter Diehr remarked that above only holds true for glass bottles. Squeezable plastic bottles can be emptied more quickly by compressing them and thus avoiding the empty volume that otherwise needs to be filled with air.
Stormwind apparently did a bit of real life experimenting which seems to support my answer as far as incompressible bottles without inserted straws or similar separate air inflows:

Upside down = 21 s, optimising air flow but bottle almost horizontal = 24 s, rotating bottle as in this answer = 14 s.

Crush the bottle fast, beginning from the bottom. The same will be true for any elastic type bottle.

It is only glass bottles that have an unsqueezable limit. I have found that if one shakes the glass bottle , the flow is faster and the bottle empties faster . Just did the experiment in the bathroom. Empties about twice as fast when shaken: slow up, fast down.

Option 3: Squeezing the bottle. Of course it depends how much pressure you are able to do, the strongest you are the fastest, and if you compress the bottle in an industrial press, the water will leave in a split second without breaking the bottle (well, it gets deformed, but that was an option).