The question above is one I started getting a lot lately. The ketubah paper is in fact ideal for all writing instruments and inks, from store brand ball-point pen to collectible fountain pen, from gel ink to india ink. That doesn't mean I don't have my favorites! Below are my recommendations:
METAL TIP (from left to right) : Jenfon ball-point pen | Pilot V5 Grip 0.7mm nib | Pilot G Tec C4 0.4mm nib | Faber Castell collectible pen - Ondoro with EF nib
FELT TIP (from left to right) : Stabilo OHPen S nib | Pigma Micron 04 nib
Metal tip pens are the most common in office and art supply stores. There are endless brands, types, thicknesses and colors. My personal favorite is the Pilot G-Tec series. Personally I advise using a metal-tip pen, because the felt-tip pens (like thin sharpies) do tend to bleed more. I did recommend two felt-tip pens I love to use; they handle writing pressure well.
It's up to you to choose which pen works best for you. Inks are no longer made with iron, so they will not discolor and fade like you see in old documents. But if you're worried, pick a pen with archival ink.
My tip is to use an old favorite pen. In your 20+ years of penmanship you'll have discovered what pen you like best. Trust your experience. For your convenience, I started giving a blank notecard with every order so you can test your pen choices (and practice your new signature).