I think it would depend on how you intend to make the physical stamp. For instance, if I am going to use a laser than just creating an image file (jpg, png, gif) would be fine since that's what the laser software can cope with easily. However, if I were going to mill it then I'd need to end up with g-code. For an image I might use Affinity Photo or Designer, for g-code I would probably draw it in TurboCAD and then generate the g-code with SheetCAM.
Of course, what you have sitting around would dictate the precise details for your usage. I suspect someone may be along shortly to suggest something like Fusion 360
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[Edit: happened while I was typing...]