legacy moveset and powerup

Invest in gengar, best ghost type and it has 15A so will deal lots of damage, don’t bother with exe unless u want to

Exeggutor is valuable right now for Kyogre raids. Gengar is amazing for what it does but if you're not interested in soloing Alakazam, Gengar or to a lesser extent Machamp, it will do nothing for you. Seeing that both have extremely good IVs and are top of their type as attackers, I'd invest into both, at least to level 30.

Absolutely! Which one to prioritize depends on which raid bosses you need help with in the near future.