which cosmic entity is the most popular and the most loved in fiction

Scarlet witch

Most popular: probably Galactus unless someone has figured someone else.
Most loved: I might say Lucifer Morningstar or Michael Demiurgos.

Probably eternity or the endless, or maybe Lucifer or other beings throughout that class

for me is phoenix because dark phoenix is the most popular story in x-men and and one of the most popular of marvel comics and 2 movie talk about the phoenix x-men last stand (garbage movie) and dark phoenix (i know everybody hates this movie but i like it) and phoenix appear also in anime

Oblivion

Possibly Spectre? I love the idea that he's not only a representation of God's wraith, he goes after criminals who have committed sins in order to avenge the victims who otherwise would have died in pain and sadness.

I would agree that it is the Phoenix Force. Just seems to be the most well known and popular. In my opinion, it has also had the most interesting stories so it is also my personal favorite.

I like infinity because she is thicc and sexy