It depends on what your focus is.  In Victor's life, he is plagued by events which pull him to and from the ancient scientists and their works--his father's and his professor's rebuking him for reading "trash"--the lightning which destroyed the tree--his beloved mother's death.

We also have his mother's benevolent nature which causes her to bring home Elizabeth.  That event is paramount, not to mention that when his mother dies, she all but makes him promise to marry Elizabeth. 

If any one of these things hadn't happened, it would have been a completely different book!

An important event that is pivotal to the plot is the time that the being seeks refuge in a hovel near the home of the French family, for it is through his association from his "kennel" with this loving and kind family that Frankenstein develops his mind and learns to speak.  In addition, there is an emotional maturation that takes place in the creature.  Without this exposure to the deLacey family, the soul of Frankenstein may not have been so nurtured.

But, as the creature remarks, "Oh, what a strange nature is knowledge."  For, with this spiritual and intellectual growth, the creature feels a need to share his feelings and desires.  It is this need for sharing that propels him into the house where the old blind father is alone.  A critical event happens here because, when Agatha and Felix return, the creature is met with fear, hatred, and horror--the same reactions of Victor, his creator.  So, the creature learns that he is destined to be alone.  And, it is because of this realization that he returns to Victor to demand that another creature like unto him be created. In this way, he can share his life with someone and not feel the terrible alienation that he does as the "miserble, unhappy wretch" that he is after his rejection by every human being with whom he has come into contact.

Along with what is posted above I think you would have to include 1) the murder of Henry Cerval, 2) the marriage of Victor and Elizabeth 3)the death of Victor, and 4) the monster's departure to the far north to die.

Frankenstein, by Mary Shelley is an excellent novel that I would highly recommend. 1)Victor enters the University of Ingolstadt and becomes obsessed with solving the mysteries of life. 2)Victor thinks he has arrived at a solution, and he begins collecting body parts in order to test his theories on a creation of his own. 3)Having successfully reanimating his creature, Victor fears his own creation, and he runs away. 4)The poor creature is left alone to wander with no instruction or training in how to live. 5)Victor receives word that his brother has been murdered. 6)Victor sees the monster in the woods where his brother was murdered, and he believes the monster did it.