Harmony of the Seas

What makes it special: Harmony of the Seas debuted in 2016 as the world's largest cruise ship at the time, and introduced Royal Caribbean's signature neighborhood concept at its grandest scale, dividing the ship into seven distinct districts including Central Park — an open-air garden with over 20,000 real plants — and the Boardwalk, a classic seaside carnival area anchored by a hand-carved carousel. She also features the Ultimate Abyss, a 10-story dry slide that was the tallest slide at sea upon debut, and the AquaTheater, a stunning stern amphitheater combining high-diving and acrobatic performances over a dedicated pool.