Here are some of my favorite inks.
Noodler's Heart of Darkness (black)
This is definitely my favorite black. Highly saturated, does not feather, and it is waterproof.
Diamine Oxblood (red)
A more classic ink with saturated dark red color. As it is nicely behaving, I love to use it in demonstrators.
Private Reserve Sherwood Green (fast dry) (green)
Beautiful dark green and well saturated ink. It shades well even with fine nibs and looks beautiful in demonstrator pens.
Private Reserve DC Supershow Violet (purple)
This ink is witchcraft. It has a unique shade and there is a noticeable shading as ink goes on paper. It is a pure joy to look while it dries.
Private Reserve DC Supershow Blue (blue)
A beautiful rich blue. It is not as fascinating as DC Super Violet but you can see it belongs to the same family.
Noodler's Walnut (brown)
Just by the color, this is the best brown to me. It is well saturated, waterproof, and has a nice flow. However, it is completely unsuitable for use in a demonstrator pen as it stains any plastic it comes in contact with. Fortunately, you can use bleach to clean the pen, so no permanent damage is done.
Oxblood heart (red)
This is a mixture of Noodler's Heart of Darkness and Diamine Oxblood (1:4 ratio). It looks very close to Oxblood in color but with just a smidgen of waterproof Noodler's. Red will still smear and disappear if you get paper wet but you will at least have thin black outline to give you idea what was written.