What Kind of Ink Is Best for Calligraphy?

The best ink for calligraphy depends on two things: the pen you're using, and what the finished piece is for.
These two questions are quite different, and it's worth thinking about them separately.

Start with the Pen

The first consideration is the type of pen, and this is mostly a question of safety rather than preference. Some inks that work beautifully in a dip pen will ruin a fountain pen.
Fountain pens have a delicate internal feed mechanism. They should only be used with inks specifically labelled as fountain pen ink. These inks are made to flow reliably through the feed without drying or clogging inside the pen.
Dip pens are far more forgiving. Because the ink sits openly on the nib rather than passing through a feed, dip pens can use a much wider range of inks - including thicker, pigmented, and waterproof inks that would damage a fountain pen.
A simple rule is worth remembering - anything labelled waterproof will ruin a fountain pen.
A useful exception worth knowing: Scribblers Calligraphy Ink is formulated to work safely in both dip pens and fountain pens, which makes it a versatile choice if you use both.

Then Think About the Purpose

Once you know which inks are safe for your pen, the next question is what the piece is for.

For Practice

If you're practising letterforms, the choice of ink isn't critical. Almost any well-flowing ink will do the job. Many calligraphers keep an inexpensive bottle of black ink for practice - Higgins Eternal Ink is a reliable choice and many tutors recommend it.
When you are practising or learning, the important thing is that the ink flows well enough to let you focus on your letters.

For Finished Work

The real decisions about ink are made when you begin working on commissions or important pieces - family trees, certificates, presentation pieces, framed work.
At this point, two qualities of the ink matter much more than they do in practice.

Lightfastness

If the finished piece will be displayed in a bright room or in direct sunlight, the ink needs to be lightfast - meaning it will resist fading over time.
This is where the distinction between dye-based and pigmented inks becomes important.
•    Dye-based inks produce vivid colours and flow beautifully, but the colour can fade when exposed to light over months and years.
•    Pigmented inks contain tiny solid particles of colour suspended in the ink. They are far more resistant to fading and are the right choice for any piece intended to last.
For a certificate, a family tree, or a presentation piece, a pigmented ink is almost always the better choice. Ziller Inks are pigmented and a popular choice among calligraphers for finished work.

Waterproofness

For pieces that might encounter moisture - hand-addressed envelopes for example - a waterproof ink is essential. A drop of rain on a beautifully addressed envelope can otherwise smudge the work in a moment.
Again, waterproof inks are safe in dip pens but should never be used in a fountain pen.

A Good Place to Start

For beginners working with a dip pen, a reliable bottle of traditional black calligraphy ink is the simplest starting point. It will handle practice happily and will produce respectable results for early finished work.
As your interest develops and your projects become more ambitious, you'll find yourself thinking more carefully about lightfastness and waterproofness. That's a natural part of the journey - and one of the small pleasures of calligraphy is discovering which inks suit which kinds of work.