Inks are by far my favorite part of all this. I am particularly fond of complex shaders like Sailor 123, 280, Haha, etc. and Birmingham Pens Eroded Bronze, Antique Sepia, and Hydrangea.
My collection of different colors has taken off after I started also painting with fountain pen inks in a watercolor style. I am also beginning to explore more shimmer inks with stub nibs, Pilot Parallels, and dip nibs for cards and holiday uses.