Which Legacy Gengar to power up?

You can't go wrong with anyone. And powering only two is pretty a waste, you should power up all of them. I have only 3 SC Gengar, 2 with mediocre IV, I did not hesitate to max all of them out.

Off-the-cuff without checking breakpoints/bulkpoints, I'd say that 3 def IVs are probably better than 2 stm IVs. The extra defense could get you to a bulkpoint at a lower level, while the extra stamina is likely only +1 HP.

I'd power up your two best ones for now, save the other for a later boost maybe.
Gengar really needs HP, and he's primarily an attack guy (IMO), so I'd rank HP & Attack as his most important IVs.

I agree with pipjay

Just make sure you get around to at least two more of them at some point. Definitely worth it

Both are close.

I'd definitely power up the 14/12/15 first. It would hit more breakpoints plus it costs slightly less dust.

Maybe power up the 13/15/13 later if you still need another ghost attacker. You can't go wrong with Shadow Claw Gengar.

13/15/13. It can survive 4 confusions from a T3 Alakazam boss at L40.
Wait, what am I talking about?