Gengar is a Ghost/Poison type which makes him weak against other Ghosts, Psychic, and Dark types. He would also be weak against Ground types, but he has the Levitate ability which makes it so Ground type moves don’t affect him.
Gengar has a high Special Attack and Speed, so I like to make him hit hard and fast! As he Mega Evolves, he is faster and hits even harder. He loses the Levitate ability and gains the Shadow Tag ability which prevents the opposing pokemon to flee. This also makes him be affected by Ground types so watch out.
The next STAB attack I would give him is Sludge Bomb (it would go good with Hex with the possibility of poisoning the opponent). The alternative would be Clear Smog (it’s a bit weaker, but if you are having trouble with an evasive or a high defensed Pokemon, then it could take care of that).
The last two choices depend on what you need. He has a pretty good variety of moves he can learn. There is Focus Blast (which is a great move to cover a bunch of different types, and it hits really hard), Dazzling Gleam (covers his Dark type weakness), Icy Wind (a little weaker but can slow them), Psychic (can lower their Special Defense), Energy Ball (another one to lower Special Defense. An alternative to this one could be Giga Drain if you want a healing move), Thunderbolt (can paralyze, which would be nice with Hex), and Hidden Power (I just really find this move to be fun).
Another set you can try that would be fun all depends on Hypnosis to put them to sleep. While they are asleep you can use Nightmare to take out a quarter of their health every turn. While they are asleep, why not use Dream Eater to heal yourself. Then finish them off with a STAB Hex that is doubled because they are asleep!