Best Gengar Attacker

do you mean worst?

The best offensive moveset for Gengar is a legacy moveset (Shadow Claw/Shadow Ball), but even with that, it's too fragile to be really useful. In this case, Dark and Figthing not only do not have overlaping SE matchups, but all of the types that are weak to dark resist figthing, so at best you'll get 1 to be SE and the other be neutral (vs Ice, steel and rock), and even then you better be good at dodging because you are gonna need it... and you won't even get STAB on either attack...

Best use would be for fighting against a normal type pokemon with normal moves, (i.e.: not Lick or Zen Headbutt) but I'd guess it is probably not enough damage to take on a Blissey with Pound in time.

Worst.

My 1st and only 91% Gengar got this too...It sits on the bench and until moves can be rerolled, it's sorta dead....I don't see Gastly ever in my area (Desert biome) so unlikely to get more anytime soon...Even Halloween had very few for me.

Maybe during Easter, we can reroll moves with candies and it will be reborn and usable.

Even with that said, I don't think the current Gengar moves are worth rerolling for until Shadow Ball comes back or moves are tweaked...due to the common current gym defenders.

Yes, sadly, this is the worst moveset Gengar can have, including Legacy movesets.