A falling raindrop has more momentum than a supertanker tied to a dock, assuming that the supertanker is not moving at all. If the supertanker were moving up and down on the waves, it would have more momentum.
The reason for this is that momentum equals mass times velocity. Of course, the supertanker has way way more mass than the raindrop. But if the supertanker is not moving at all, then its velocity is 0. Therefore its momentum will be 0. The raindrop may not have much mass, but it has velocity and so its momentum will be more than 0.
For security reasons it may be better to presume that super tanker can impart a very high momentum. The reason is you have not told many big things but only a small negligible information about the rain drop.
But for theoretical purpose rain drop for its little mass and velocity has at least some momentum as against zero momentum of a supertanker of zero velocity. The measurement of minute velocity to a perfection may be of some difficulty.
A super tanker is always exposed to the forces of (1) wind and (ii) waves of the ocean, which is more probable than a very less probable event of rain drop.
Taking into considerations more probable major events and their mass and velocity the expected momentum of a supertanker is more than the rain drop.
Momentum of a body is defined as mass multiplied by its velocity. An body has stationery it has zero velocity and irrespective of its mass the body, its momentum is zero.
If we assume that the super tanker tied to dock is stationery, then its momentum will be zero, and a falling raindrop, in spite of its very small sizes and mass will have a small positive momentum. Therefore it will have momentum greater than that of the super tanker.
But the reality is that the super tanker in reality will have some oscillating or vibrating motion because of action of waves an water and other disturbances. This motion will be too small to be felt, but it will be there. Also, being oscillating motion the super tanker will not have a uniform velocity. Instead it will have velocity which changes in magnitude as well as direction. thus there may be instants when the super tanker has no velocity at all. At other times it will have some positive velocity. Thus the tanker will have a variable momentum. However chances are that average momentum of the tanker will be much more than that of a falling raindrop.
The raindrop is falling and has momentum p = mv where m is the mass and v is it's velocity. If the supertanker also has momentum equal to p = mv but it's velocity is 0 (zero) then it has no momentum because m(0) = 0