Gonna go with Strong Force Manipulation because dr. manhattan is a user.

Pretty sure gravity is the weakest among them.

Electro

While the strong force would be the most effective in a fight as it lets you disintegrate objects and cause nuclear explosions at will, electromagnetism and gravity are definitely the most versatile:

The former encompasses electrical phenomena, magnetic force, all contact forces (normal force, impact force, friction, pressure, compression, tension, etc.), radiation (a.k.a. light) and, get this, chemistry (chemistry is just electromagnetic interactions at the atomic scale) as well,

while the latter pretty much just describes how the shape of space-time affects the movement of matter. In other words, there's no real difference between Gravity Manipulation and Space-Time Manipulation.

My personal pick is Gravity as it strikes the perfect balance between versatility, power and sheer scale.

Edit: Actually, scratch my initial statement. Gravity and Electromagnetism can be just as effective as the Strong Force in a fight. The gravity user could collapse the target's body into a gravitational singularity (a black hole) while the electromagnetism user could instantly shut down the target's brain by stopping all electrochemical activity within. The way I see it, the fight would end instantly with all four dying at the same time, none of them from the Weak Force, thus in a three-way tie between the Strong Force, Gravity and Electromagnetism.