One of my preferred inks, particularly for use in vintage fountain pens, is Waterman ink. Vintage pens require care and maintenance, which includes regular flushing. Since vintage pen mechanisms can sometimes be complicated, depending on the filling system, and not easy (or recommended) to disassemble, ink that flushes easily from the pen is preferable. Waterman inks behave well; they have good flow, wash out easily, are relatively non-staining, and are non-pigmented. In other words: they are safe for your fountain pen. Responsible vintage pen ownership involves caring for your pens.
What inks do you like to use?
Is there a Waterman ink you favor?