Rifle Paper Co. Pen Refills
I searched for months and spent way more on refills that I'd care to admit, but I finally found a refill I'm happy to put my recommendation behind.
The Schmidt Easyflow 9000 writes beautifully. The black is BLACK (I've not tried the blue, but have heard it is vibrant). It gives a really smooth writing experience with a nice feedback. I've not yet had one skip on me.
I found them on Amazon via this link right here.
As I find more that make the cut, I'll continue to add them to this page.