If you are looking for a fraud resistant ink, any of the Noodler's Bulletproof or eternal inks will do quite well. Others have mentioned Legal Lapis and Luxury Blue, I have both. You may also want to consider Noodler's Brown or Verdun Green, both will definitely stand out on the original document.

The one caution I was given regarding these inks is to remember to flush out your pen regularly (I do it when I refill). In general, the old addage "cleanliness is next to godliness" also holds true for fountain pens.

You can get a good idea of what a large number of inks look like, and how well they work over on the Fountain Pen Network. Here's the link to the ink reviews:

http://www.fountainpennetwork.com/forum/index.php?showtopic=39354