There isn't one.

Or, if you must have a default, "auto" would work better than any particular value.

Even ignoring the demands of different lighting conditions and focal lengths, the value you most often use will depend on your personal preferences: whether you prefer a small or a large depth of field, whether you prefer fast or slow shutter speed, whether you prefer low noise or low background blur.

Usually, the most useful range of a lens is from its widest aperture through to about f/8 (on full-frame). Smaller than that, ie f/11 through to f/36, usually doesn't give you much benefit and starts to introduce softness through aperture diffraction. If you need to cut down light this much, it's often (but not always) a good idea to use an ND filter instead.

I'll say what I usually say to people who ask what the best "manual" camera settings are: leave your camera on auto. If you don't know you want a certain aperture, or shutter speed, then you may as well use auto and then go with whatever your camera chooses. Photographers set the aperture or shutter speed when they know better than the camera about what aperture or shutter speed they want to use. If you don't have a strong preference, use auto as a starting point and venture into aperture priority when you have a specific requirement, such as background blur, fast speed, or so on. Using aperture priority mode when you don't know what aperture you want doesn't make sense.