https://www.smogon.com/forums/threads/usm-ubers-viability-ranking-thread.3623296/
According to the viability ranking, Mega Gengar has higher rank than Aegislash. Gengar being ranked S while Aegislash only C. Based on their smogon analysis page, Aegislash, while strong, is very prone to being worn down and needs more timing to pull off because of how easy it is to kill if it is in blade form. With Gengar, the only caution is to time its switches because this thing dies to nearly everything ubers throw at it. Mega Gengar also has the dreaded shadow tag and ubers has adopted to using shed shell just so they don't die to it
You can read about them in further detail here
https://www.smogon.com/dex/sm/pokemon/gengar/ https://www.smogon.com/dex/sm/pokemon/aegislash/
