Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-2189621-20160510231634

I would've said Flowey, due to his abilities to SAVE, reset, and manipulate, but I realized Chara has all of those abilities and more. Plus, he killed Sans, something Flowey couldn't do, which resulted in him striking fear into Flowey himself.

So, Chara, definitely.