It’s the first one!
Dr Mehmet Oz says that the most common behaviour of someone going to use a cubicle in a public bathroom is to skip the first one and then use a clean one further down.
But because of this - the toilet least used is the first one!
The reason people skip the first stall is thought to be because of privacy, finding a spot further back in the room might be more comfortable.
Or, people might skip it assuming it is more commonly used because people will choose it for its convenience.
So if you have the choice, go in the first cubicle. Science says it’s the most hygienic.