Here's the order Harry destroys them:
1. Diary of Tom Riddle (in Chamber of Secrets): This was the first Horcrux destroyed.
2. Ravenclaw's Diadem (in Room of Requirement): This was the second Horcrux destroyed.
3. Hufflepuff's Cup (in Gringotts): This was the third Horcrux destroyed.
4. Salazar Slytherin's Locket (in the cave): This was the fourth Horcrux destroyed.
5. Harry Potter (inside himself): This was the fifth Horcrux destroyed.
6. Nagini (in the Forbidden Forest): This was the sixth Horcrux destroyed.
7. Tom Riddle's Body (in the Great Hall): This was the final Horcrux destroyed.