There’s nothing that I can remember reading that suggested Snape was the #1 Death Eater. In fact, there’s more evidence to point that the Lestranges were at the top.
The Dark Lord will rise again, Crouch! Throw us into Azkaban; we will wait! He will rise again and will come for us, he will reward us beyond any of his other supporters! We alone were faithful! We alone tried to find him!"
—Bellatrix Lestrange on trial for her Death Eater activities
“Voldemort’s fury at the fall of his last, best lieutenant exploded
with the force of a bomb.” - Death of Bellatrix
So Bellatrix at least believed herself to be top of the list with her husband and Voldemort held her in high regard. This is further demonstrated as potentially being true as Lord Voldemort chose the Lestrange vault to hide one of his Horcruxes in.
I think we can safely say that Snape was not a top Death Eater during Voldemort’s first reign. His news of the prophecy may have ingratiated him to the Dark Lord, but not enough to put him in the top spot. He was loyal, to a point. We know Snape loved Lilly. Love is the one thing that Albus keeps saying is the most powerful form of magic. It seems it was enough to break ANY alliance Snape had to Voldemort.
Also, I think Snape may have actually begged for the life of Lilly Evans. Voldemort seemed to not wish to kill her if it could be helped.
"Step aside, woman!" said the shrill voice of burning cold. "For you I
am not come, only the boy." "Not Harry! Please… have mercy… have
mercy…" Lily Potter, Harry thought, seemed not to understand what
type of people became Dark Lords in the first place; and if this was
the best strategy she could conceive to save her child’s life, that
was her final failure as a mother.
"I give you this rare chance to
flee," said the shrill voice. "But I will not trouble myself to subdue
you, and your death here will not save your child. Step aside, foolish
woman, if you have any sense in you at all!”
And before that we see that he came right in and killed James, without hesitation.