Bottom-left

Top right personally

Top right.

Top left.

Top right is slick

I like bottom left

I'd go for top right

An all black top right for night achievers

For the super-simple app, I'd go with the top right. Just enough visual clues to convey the message and remain simple

Top left is more recognizable, top right feels like native iOS, bottom right is more elegant with its black color dominance. These design choices are hard, mostly subjective, and reflect the creator's taste.

Just installed the test flight and trying the app. Seems very nice so far! The icon it now has (top right) has to compete with a lot in my dock, so the better choice in my opinion would be bottom left.

Top left