This is the million dollar question. So many different inks, so many different applications.
If I'm using rubber stamps, I like Memento. If I'm using clear stamps, I like the dye ink pads from Clearsnap or the new Stampin' Up! formulation. If I'm just doing any kind of water color stuff, I like Ranger Distress. If I'm coloring with Copic, it's Espresso Truffle Memento for me.