Definitely Aizen. Way more smart, has lots of spiritual energy based moves like Kido and others, his Hogyoku awakened form, and his abilities like Kyoka Suigetsu. Without a doubt, Sosuke Aizen.
Shang Tsung doesn’t even have a weapon that could kill a shinigami let alone fight one, Zanpakutō are weapons that attack souls so all Aizen needs to do is cut him or use his spiritual pressure and he is dead, just spiritual pressure is needed to kill Shang Tsung
