Favorite Fanfiction?
Several. There's one called My Brother which is basically a "what if Ken's brother lived?" type thing. Another one is "Youngertale" which is the cast minus Mettaton and Muffet all getting younger (at this time. That could change) and then there's all the Yokai watch fanfictions.
Favorite Headcanon?
Currently its one of http-Berkut FE echoes ones involving the whole thing being Mila and Duma playing with toys.
Favorite Undertale character?

Favorite Oc?
Either Fates!Ariel, Rontao or Shizen. I might be making kids for Berkut and Rinea.

Favorite YouTuber?
I have a list

Solar Sands
AludinGinka
Man on the Internet
SlimKirby
Constructive Criticism

And that's not even the full list