The protagonist of Mary Shelley's novel Frankenstein can be confusing given the use of the multiple narrative voice seen in the novel. For some, the main speaker of any given work is seen as the protagonist. In this case, Walton would be considered the protagonist (given he is telling Victor's tale).
That said, some may argue that Victor is the protagonist of the novel. The story encompasses Victor's life, loves, and conflicts. The story would not exist if it were not for Victor's experimentation with reanimating life and his success at doing so.
Others may believe that the Creature is the protagonist. This can come about given some readers may feel sympathy for the abandoned "son" of Victor.
Essentially, an individual reader could successfully justify the protagonist of the novel as any of the three "main" characters of the novel. With each declaration of the protagonist, the antagonist changes. If Walton is defined as the protagonist, the antagonists are nature (external conflict) and himself (internal conflict). If Victor is deemed the protagonist, both the Creature (external conflict) and himself (internal conflict) would be the antagonists. If the Creature is deemed the protagonist, Victor (external conflict), society/mankind (external conflict) and himself (internal conflict) could be considered his antagonists.