Well, my fountain pen preferences change often enough to make whatever I say today obsolete by the time you read it. 
However, I have been experimenting with making my own iron gall ink from a medieval recipe I found. There's a park nearby where I collect the oak galls, and also some reeds from the pond to make reed pens (you DON'T want to use iron gall ink in a fountain pen!). I use the ink with glass and steel nibbed dip pens as well. I've also made india ink from lampblack soot and gum arabic, but the iron gall ink can come out in beautiful shades of red and brown, depending on the recipe.
There's something about writing with ink and pen that you made yourself. Next step is to make paper for a totally self-sufficient writing experience.