Pencil if the medium is matte. Go to a gallery or art museum and look; virtually all will be signed in pencil. It's the most archival writing instrument because unlike all forms of ink, pencil will never, ever fade. The graphite will literally outlast the paper itself. Also unlike many inks, pencil will not damage the paper because it has no chemicals in it acid or otherwise. It will smudge if rubbed, but you shouldn't touch the surface of a print anyway.
A fine or brush tip are excellent choices as long as the ink or pigments are archival. A few pens out there that will do a great job and are used for proffesional grade: Copic Multiliner SP Rotring Rapidograph Sakura Pigma Micron Prismacolor Premier Illustration Markers Uni Pin Fineliner Faber Castell Staedtler Pigment Liner Sharpie Permanent Marker fine tip or brush tip We include a paper sample so you can give a try before deciding. Ballpoint or gel pens can be used if your hand is more used to it, but make sure they are archival and text first on the paper sample included.