Determining the "largest" ship can be subjective, as it depends on the criteria used (length, weight, cargo capacity, etc.).
Prelude FLNG:
This floating liquefied natural gas (FLNG) facility is the largest ship ever built. It's essentially a floating gas processing plant.
Pioneering Spirit:
While not as long as some others, this construction vessel is the largest ship in terms of weight and lifting capacity. It's used for installing and removing massive offshore.
Seawise Giant: Although no longer in operation, it was once the largest oil tanker ever built. Its size and capacity were legendary.
Valemax: These bulk carriers are among the largest ships dedicated to transporting iron ore. They are colossal in size.
Oasis-class cruise ships: These cruise ships are some of the largest passenger vessels ever constructed, offering incredible amenities and capacity.