If you could only ban one God..

She is already picked/banned in probably >90% of league games and you want to give her more damage?

At the moment, Chang'e is top ban. Zeus does stupid damage and a good Merc is gg when even a little fed (even more than other hyper carries,) but those two are easier to deal with than Chang'E. One mistake against her is enough to snowball her to the point where even camping her doesn't matter because she'll kill at least one person in the 1v2 before she dies.

Arachne is a good one too, just because she's so hard to deal with in solo queue.

Arachne tends to dominate more in the solo queue where she preys upon the enemy team's lack of coordination. Step into premades, however, and she's much less effective because people there are typically coordinated enough to shut her down before she can become a threat. Not having a proper escape move means she's easy to shut down in that regard.

It also helps to try to drag a match out. Arachne doesn't fare too well in the lategame, and must rely on pulling people with Cocoon to accomplish anything. If she misses, she's useless for a good twenty seconds because she usually won't survive in the middle of a teamfight.

I guess you didn't know that I was joking about chang'e