It took harry a long time to find out how to destroy it

nagini or harry

She was always with Voldemort.

to destroy this (he thought) he had to get killed

Nagini is always with Voldemort, so she's the most difficult to destroy. Harry has to be easier to destroy xD

among all these...he is the most difficult to b destroyed...he was always being protected by that old man (i juz dun 1 2 type his name here)

They were all so difficult to destroy and they brought such big consequences, like Dumbledore's death and his black arm, the near death of the trio and the death of Goyle (even though I'm not so sad about that part), the big escape from Gringotts, where they almost got caught and lots of other stuff....

Harry had to make the decision to have himself destroyed pretty much. As a moral decision, this one was probably the most difficult since it required the most sacrifice on his part, and he had to go through a lot to get to that knowledge of his imminent martyrdom, so couple that with the act of actually going through with it, he had to do a lot to get to that point.

They were stuck with that thing forever!!!

Given that this one involved Harry having to be willing to sacrifice himself. If Harry wasn't listed as an option, I'd have voted for the ring, simply because of its consequences.