If you're asking this because you're thinking "I'll GM whichever tradeskill is most needed", I'd encourage you to give up that train of thought. P99 has been around long enough that there are lots of GMs of every tradeskill around already (and you can see as much in a
stickied thread in the EC Tunnel forum). There
might be a cultural smithing one that doesn't have
that many GMs already, but then there isn't
that much demand for cultural armor.
Instead, I'd advise picking the one you most want to do; there will be others who have mastered it already, but who cares?