I've tried quite a few inks, and my favorite has to be Aurora's blue ink, which approaches blue-black. It's a joy to write with, no matter the pen (some pens seem to be a bit particular about their ink). Nice color, too.

If you're going to be signing the next Declaration of Independence, and are concerned about the signature lasting, not being tampered with, etcetera, there are some specially made inks - Noodler's. I've heard great things, but haven't gotten around to trying it myself. Better yet, use India ink with a dip pen, which I think is probably as archival as you can get. Just don't use it in a fountain pen - you'll probably ruin the pen!

If the pen, rather than the document is the priceless object, common advice is to stick to Parker Quink and Schaeffer Skrip, which are pretty mild inks, and less likely to dye a clear ink view window, for example.