My best guess is that it was Peter. Wormtail. Because if I was Lord Voldemort, i would make DAMN sure I had Wormtail under supervision. A man who turns his coat will surely turn it again, and all that. If he went straight to Voldie after he was made secret keeper, Voldemort would want to go out as soon as possible to get in his 'killins'. But he'd also probably want Peter there, to make sure Peter was telling him the truth. Also here woul be the added bonu of james/Lily taking down wards and letting them in, because they assumed it was just Peter, and not the most evil wizard in the world....
Hang on a minute, if Snape had already turned against LV before his downfall, but was a double agent rather than leaving the DE's altogether, why didn't he warn everyone that Peter was a traitor? He never even told DD about Peter beng a DE, that seems like something he would have divuldged if he was truly reformed.
Ah, but Peter was weak and very cowardly. He wouldn't be in the inner circle, now would he be? And the only time we ever saw Death Eaters being named was when Voldemort was adressing the inner circle in GoF And remember Karkaroff saying in book 4 that no one knew the names of every Death Eater? Snape wouldn't have been able to be sure that Sirius was or was not a Death Eater, same with Peter.
And as for Snape being at Godrics Hollow: watching the Potters be killed wasn't what made him turn. he turned way before that, at lthe very least 1 month before: remember, he was teaching at Hogwarts before then. Also, the prophecy was made a year before Harry was born, and his parents were killed a year AFTER he was born: It wouldn' t take Voldie two years to figure out what the prophecy meant, and since it was the realization that he'd doomed Lily and James that made Snape turn....he was on our side way before then.