I've been trying a lot of different inks over the past few years, and I've definitely settled on a brand I'm very happy with. And that brand is Holbein.
I first heard about Holbein ink on the Comic Tools site. Comic Tools compared the Holbein ink to Dr. PH Marten's Hi-Carb ink, but I've tried both and found the Holbein to be far darker. And darkness — the blackness and opacity of the ink — is key to me.
I ink pretty much exclusively in brush. Most inks I've tried are watery and end up drying gray, which makes for an unsatisfying finished drawing. It's kind of shocking how many inks do this and require several coats to build up a rich, completely opaque black. Even the big names have this tendency, from Higgins to Speedball.
But the Holbein, when thoroughly shaken, coats well and creates a rich, deep, opaque black on the first try most of the time. Occasionally a large dark area will need some going over, but for the most part, the Holbein black is very solid. It also flows well from a brush, though after long periods of use the ink can start to get thick and gum up the brush a bit. A little water usually fixes this right up, though every now and then, after a particularly long inking stint, I need to completely clean the brush to get it working properly again.
Holbein ink actually comes in two flavors: standard, which is your typical shellac-based ink, and what they call "Special," which is acrylic. Being something of a purist, I've mostly been using the standard type, but recently decided to try out the Special.
The Special ink is a bit thicker than the standard, and so it dries quicker and tends to gum up faster, but boy is it black. This is the blackest ink I've ever tried, and for large areas of black it can't be beat.
What I've started doing is using the standard ink for art that's smaller and/or mostly linear, but for larger pieces with a lot of black, I've been using the Special ink, and I'm pretty sold on it.
The Holbein inks — both standard and Special — are superior inks, producing rich, opaque blacks far better than any other ink I've tried. I recommend either, or, if you're like me, both.
UPDATE:
Still loving the Holbein, but I have a new contender for best ink. Read about it here!