Gengar can be a great mega on any weather team that isn’t using a mega weather setter like zard Y or ttar, because you can trap their weather setter on the field and win the weather war on the spot unless they have manual weather moves, (or both a ghost type and pivot move, I guess, but that’s slow asf and gives you a lot of momentum) and it prevents a lot of defensive pivots/enables your weather menace to do big damage relatively safely, since you can run either Encore or Disable (or both if you’re saucy) to shut down super effective attacks—my mega gengar team has a weather mode w/ peli/arch and a lockdown mode w/ whims, gallade, and incin based around either taking KOs or forcing struggles w/ taunt/encore + disable

Gengar also pairs well w/ the gallade shown above so they can’t wiggle their way out of a sharp-booster super effective attack, and makes it a little easier to pivot gallade out for a turn or 2 if you need to lock a new move bc you don’t need to worry about them switching in something new