Mega-Gengar is less about synergy and more about clearing the field for one of your sweepers. As ReadyAimFire stated in his comment:
You don't need good synergy. You just need something that HAS to get
rid of something else, so if you were using say Groundon, you'd add in
MGengar to take out Kyorge for example.
commented 4 hours ago by ReadyAimFire
So if you have a team consisting of Mega-Gengar, Xern and four others, then you're using Gengar to eliminate any checks or counters to Xerneas, such as Bronzong, Jirachi, Scizor etc. Mega-Gengar will not be switching into very many attacks at all (like none because just about everything neutral+ 2HKOs it) so you're less worried about synergy than you are about your sweepers and if MGar can really support them well. Generally you want sweepers that require little set up such as EKiller, GeoXern, Calm Mind Kyogre (Once it's counters are cleared it's easy to set up), Dancing Groudon, and so on.
If you're asking about in-game ignore what I said and use whatever Pokemon you think is cool 0.0!