Board Thread:Questions and Answers/@comment-27327020-20160404174206/@comment-13682699-20170409155306

Asriel as the God of Hyperdeath possessing all the equivalent of 7 souls is by definition the strongest being in existence and can. The only thing that would be considered above him would be Annoying Dog (a.k.a. Toby Fox himself) as the creator of the Undertale Universe itself.

Flowey with 6 souls is then the default next strongest being with only one less human soul (or the equivalent) than the God of Hyperdeath.

The next strongest after that would be Frisk/Chara considering they share the same Soul to use their power. By having the highest determination of everyone, they possess the most potential power in a single being excluding external assistance and can Reset/Save/Load/Erase the world so long as no other being has more total net determination than them, which in itself is incredibly high considering the God of Hyperdeath had to Erase them in order to gain full control of the timeline.