These are the top five strongest force users in Star Wars canon. This is all my opinion of course, and not necessarily in order as I could change my mind and some of them could be interchangeable:

Emperor Palpatine (Darth Sidious)

The most powerful Sith Lord in galactic history. Emperor Palpatine aka Darth Sidious possesses complete and total mastery over the dark side of the Force. His deep understanding and knowledge of the Force makes him a nearly unstoppable threat.

Luke Skywalker

As the son of the Chosen One, Luke has an unbelievably strong connection to the force. Despite a late start to his training, he goes on to become the strongest Jedi of all time, acting as the ultimate instrument of the light side of the Force.

Darth Vader (Anakin Skywalker)

The Chosen One that was conceived by the Force to bring balance, Darth Vader aka Anakin Skywalker has the highest midi-chlorian count ever recorded and the strongest connection to the force of anyone in galactic history. He would have easily become the most powerful force user of all time if it weren't for his injuries and emotional conflict. Despite this, he is still unbelievably powerful; a master of the dark side and an unparalleled lightsaber duelist with cybernetically augmented strength and durability.

Yoda

The Grandmaster of the Jedi Order, Master Yoda has one of the highest midi-chlorian counts ever recorded, and with centuries of experience and training he has total mastery of the lightsaber and the light side of the Force. He is undoubtably one of the strongest Jedi of all time.

Obi-Wan (Ben) Kenobi

In spite of his relatively average natural connection to the force, Obi-Wan Kenobi still became one of the greatest Jedi of all time through hard work and determination. He is the best defensive lightsaber duelist ever, and his desire to protect and utter devotion to the will of the Force gives him great power.