Nagini, because she was the last horcrux that Voldemort made. He did not know that he already had seven horcruxes (including himself), so he made Nagini another one. He had 8 horcruxes instead of the 7 he intended to have. 8 is not a magically powerful number. Making Nagini a horcrux was the cause of Voldemort's downfall.

But I thought that he technically he made Harry after nagni so That logic would mean its Harry.

No, Nagini was not made into a horcrux until after Harry freed her from the zoo and she found Voldemort. Voldemort did not know that Harry was a horcrux.

No, it was never said that that was the same snake at all. It was just a theroy. The snake he set free was a Boa Constrictor which is nonvenomous. well the one voldemort had was said to be venomous and was a type of viper. After you said that, I looked up information on it.

No, it says she was made a horcrux after Harry was. She became a horcrux after she killed Bertha Jorkins. That happens right before the Goblet of Fire takes place. Harry became a horcrux when Voldemort tried to kill him when he was a baby.

It says Nagini was made a Horcrux in 1994. But Harry became a Horcrux in 1981. Which means, Nagini was the last Horcrux.

Yea you guys are right. But she still wasn’t the one Harry let lose. I was reading about how voldemort used her to comeback to power so I assumed it meant that was like right after Harry.

J.K Rowling has said that it was Nagini that Harry freed, but it kinda does make more sense that it wasn't the same snake once you explained it.

JKR naver said it was the same snake!! She said she has no problem if people believe they both are same.