Most commonly it is pickled herring. I don't think rotten herring is a thing in any broader circles.

Homemade bäsk is usually much better than factory made Bäska Droppar, if you enjoy the taste of wormwood more than just being slapped in the face with artificial bitterness and sugar.

Of course, if you hate the taste of fish, pickled things and spirits in general, you are unlikely to enjoy any of it.