Sailor Studio 027
While I love trying different inks (it's fun and, let's face it, not all inks and fountain pens get along well so ink diversity is important), Sailor Ink Studio is quickly becoming my go-to ink for this reason: it dries quickly and it doesn't smudge when using a highlighter over it--IMMEDIATELY AFTER IT DRIES. I want to emphasize this because of how rare this is. I've been using fountain pens pretty much exclusively since the summer of 2015 and I have never found an ink that can stand up to my color-coded world like Sailor Ink Studio. I am not exaggerating when I say that I can, quite literally, write something on the page and highlight it as soon as the ink is dry (which again, is pretty darn quick) and it won't smudge. For anyone who is in school or is at a job where highlighting their notes is important to them, I HIGHLY recommend this ink family. I would venture a caution that, like all other things in life, there will be an exception or two within this line of inks and some color or other will smudge when highlighted.
Another reason I love these inks is the depth of color, even in the lighter hues. Dye-based inks overall tend to have more depth and subtlety. This particular color doesn't really fall into the subtle/shading category because it's a black--but it's a lovely, smooth, rich black that sits primly and cleanly on the page regardless of the quality of the paper. I'm using 027 in my new Pilot Vanishing Point and it's a marriage of both love and convenience for me in my professional life.
If you haven't tried Sailor, you need to buy one of these lovely inks today! You'll have a hard time going back to the cheaper inks after you have, but since 20 mL of ink actually lasts quite a long time--even for someone who writes a lot--the $18 you spend will bring you months and months of writing pleasure.