The particles on the surface of a liquid drop has maximum potential energy. The more the surface area, more are the particles on the surface and more is the potential energy. As a rule, any body tends to be as stable as possible. The body having more potential energy is more unstable. For a given volume, a sphere has minimum surface area. Hence, drops acquire spherical shape to acquire minimum potential energy and more stability.

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because of surface tension, every thing in the universe wants to achieve stability,they do that by lowereing their energy.which in this case is the surface energy.a sphere offers lowest surface area yet a very large volume .

The force of attraction between the molecules of water is higher.

Te force of attraction between the molecules of water is higher then gases but lesser than solids.usually it spreads along the container in which water is kept,but during free fall as it occurs to rain drop,the forces tend to exert a combined effort to pull up to a shape which has smallest surface to volume ratio for a specific volume and that shape is our well known SPHERE!!!!

If the drop is small enough, it is a perfect sphere. A sphere is the geometrical shape that has the smallest surface area for its volume. The drop takes this shape because water molecules tend to stick to each other [because of there polar covalent bonds]. So, when not confined by a container, and with nothing around it to distort its shape, a very tiny water drop is perfectly round like a ball because the water molecules are pulling inward toward each other.

If the drop is larger like a raindrop in free-fall, it has a domed top and a semi-flattened
bottom because as it falls it must push the air out of its way. That "upward" push of the
air being displaced causes the falling drop to have a rather flattened bottom.

Contrary to popular misconception, a free-falling raindrop is not shaped like a teardrop --

round on the bottom and pointy on top. 

water(liquid ) property Surface Tension . Surface tension is a property of the surface of a liquid that allows it to resist an external force.The only possible shape is spherical That can be resist the external force means forces all acting on outer side of the droplet that equals to the sum of all forces that binds together the droplet is leads to acquire the spherical shape.

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If the drop is small enough, it is a perfect sphere. A sphere is the geometrical shape that has the smallest surface area for its volume. The drop takes this shape because water molecules tend to stick to each other [because of there polar covalent bonds]. So, when not confined by a container, and with nothing around it to distort its shape, a very tiny water drop is perfectly round like a ball because the water molecules are pulling inward toward each other.

If the drop is larger like a raindrop in free-fall, it has a domed top and a semi-flattened
bottom because as it falls it must push the air out of its way. That "upward" push of the
air being displaced causes the falling drop to have a rather flattened bottom.

Contrary to popular misconception, a free-falling raindrop is not shaped like a teardrop --
round on the bottom and pointy on top.

This is due to the surface tension  of water.

Because of the surface tension , the drop tends to have minimum possible surface area, and the shape which is possible with  minimum surface area is  "spherical shape". 

Hope its clear to you.