Noodler's Ink: my favorite ink for sketching
I used to hate sketching in ink because I couldn't erase my mistakes. Then I started to appreciate those extra little lines and marks that wouldn't go away, and even noticed that I had fewer and fewer of them because my hands learned to follow my eye more closely.
Because I sometimes like to add watercolor to my drawings, I needed an ink that did not bleed even if the paper wasn't super duper fancy. I started asking around other artist who had similar sketching habits about their choice of ink, and their answers were all the same: Noodler's Ink. Relatively cheap (it lasts a long time for my needs) and consistently bleed proof, the "Bulletproof" is a great option for both sketching and nice pen and ink drawings.
The pigmented inks are also amazing but WILL bleed when exposed to wet media or water which I find a nice option to have, personally.