That is not an easy question, nor is it that easy to answer without knowing the abilities and preferences of the to be 3d artist.
For someone coming from a classic photographic background it would be lighting.
Someone with an more mathematical sense i guess would tend more to shading/rendering (or rigging and simulations)
Someone who can draw good can easily learn 3d painting.
Animation, VFX, character design, rigging are the more difficult fields.
Abstract Motion graphics is easy to get into, but takes a while until good stuff comes out.
I would suggest NOT to look for the easy road.
It mostly leads nowhere.
Things are not easy or get easier, you just get better and faster.