I've seen Uni-ball 207 recommended too. I also have some Mont Blanc fountain pen ink that claims to forgery resistant (meeting ISO 14145-2) that I've used to for check writing. Here's a YouTube video about the subject [1].

Noodlers ink (also available on jetpens.com) is said to have forgery resistant inks as well. I don't have any of it, but I do have some of Noodlers invisible ink--you need an ultraviolet light source to see it.

[1] https://www.youtube.com/watch?v=tH_Yb4-0p0o

Gel pens in general are recommended over ballpoint, because the ink is water-based and resistant to solvents.