Cancun, no doubt. I was about sixteenish at the time, and got to stay at an all-inclusive resort, lounging on the beach, swam with a couple of dolphins, took a tour of the Mayan ruins of Chichen Itza, ate at multiple completely-free all-you-can-eat buffets, got propositioned by a woman, and lounged on the fine sandy beaches as the warm waters of the Caribbean came rolling in.
It was luxurious and easily the best vacation I've been on to date.
(And no, I did not take the woman up on her offer)