Black ink, of course. Been using it for a few years now, nothing else comes close. (Well, the Uni-ball Signo RT 0.38mm comes close.) Anyway, if you’re not buying pens from JetPens, your pen probably sucks.
[Well I haven't bought from them (yet) but my favorite pen is the Pilot Razor Point II, super fine, in black (natch). It does not suck. 9That's for general use (daily scribbling with a touch of sketching and whatnot.) Other uses demand other pens...]
Source: Daring Fireball