Waterman Blue Black has always been one of my go-to inks. It and other blue-black inks I reguarly use are in Inks of Note - Blue Black
On one of visits to Paris I stopped by the Louis Vuitton store and picked up some of their ink. Expensive, but beautifully packaged. Good Ink also.
Brown and Blue inks in Inks of Note.
The Graf von Faber-Castel line of ink is very good. Cobalt Blue is one of my favorites.
The iroshizuku line of inks are so popular, as they should be. Overall, excellent inks.
One of the advantages of using a fountain pen is the wide selection of inks. My pen cabinet is a testimony to the saying: "You can never have too much writing ink!"
Some of the inks that I use become my inks of choice, or a ink to take note of. These are the inks that I find I keep cycling through my pens on the most frequent basis.
I think to be a great ink, it has look good when you are using it, and also, when you go back to read something you have written some time later. I have had may share of experiences with inks that on one day look great, but when I go back at a later time, I have to ask myself: “What were you thinking!”
Quantity or mass of writing is also a test. Just about any colour looks great for a single line, a quick note on a card. When look at an entire page of writing and some of the colours just don´t look so good.
I tend to lean towards bold colours, colours that have a presence on the page, inks with solid saturation of tone, so with a broad or stub nib the mid-section of the stroke is not washed out.
The actual tone of ink will vary depending on the nib used in terms of the width of the stroke, the pressure used to write, pressing harder creates more ink flow, the flow of the pen itself impacting the amount of ink hits the paper, and the characteristics of the paper in terms of absorption or feathering.
Enjoy your ink, and express yourself.