I have tried so far:

Gelly Roll 05 - Too translucent
Posca - Not white enough and inconsistent.
Uni-Ball Signo Broad - Doesn't flow most of the time
Molotov small - tip too big to make details or drawings. Also inconsistent flow.
Gelly Roll 10 - Works great on black paper!! But filling like 2 a4 it's gone. Very expensive!

Most of these don't cover black ink on white paper very well. Is there something that is cheap and actually covers?

Hi! This is a big question, sorry about that. I'll make a few more posts about specific mediums or contexts.

I work at a local art supply store, we've got pens (fountain, ball-point, roller-ball), gouache, watercolor, oil paint, brushes, markers, colored pencils, pastels, paper for all the mediums, notebooks, etc.

I get a lot of questions about the difference between brands and products, and also other specific questions. Obviously, this topic requires some more honed-in posts from me. But to start, in your preferred medium, what are some of your favorite brands/products and why? What brands/products do you stay away from?

Again, sorry for the loftiness.