Some artists on pixiv go to great lengths to ensure their art depicts a sense of realism. For example, illustrations featuring characters with exposed torsos typically give a sense of the smoothness of their skin. Detailed lighting effects help bring the entire area to life from the curves to the navel.
Baltimore (from Azur Lane) has a terrifically toned tummy, doesn't she?
Have a look at the fabulous abdominals below and see which ones give you butterflies.