Although she maintains a secure status as a primary member of the DC Trinity, Diana Prince's rogues gallery arguably isn't quite as dense as Batman's or Superman's. Having said that, she still has a few iconic foes -- all of whom could've made great antagonists for Patty Jenkins' first Wonder Woman movie. With that in mind, it raises a question: why did the film opt for Ares? According to comments made by Jenkins in the Wonder Woman home release's special features, she had to use the God of War because he's the quintessential Wonder Woman villain, saying:
In my opinion, it would've been a mistake to make a first Wonder Woman film without her absolute arch rival nemesis Ares, who is the most classic villain from the lore and is her counterpart of her point of view. She's a god and he's a god, and he knows something she doesn't know and made a choice based on that. He saw the weakness in his father's creation and is trying to show the world how bad mankind is and therefore annihilate them and get rid of them. She, in the course of her journey, learns the same thing and ends up saying 'oh my god they are all of those things,' but she makes the opposite choice. If her story is about a shift in point of view, his storyline is a participant with that story, instead of being a villain bad guy, which is the point of the movie.
This rationale certainly seems to make sense when we look at the history of the modern comic book movie genre. Tim Burton's Batman used The Joker, Richard Donner's Superman used Lex Luthor, and Sam Raimi's Spider-Man used Green Goblin. It's important to use these arch villains because their stark contrasts to the heroes help define the protagonists, and their franchises as a whole. By introducing Ares and establishing a particular outlook on humanity's inclination towards conflict, Wonder Woman simultaneously manages to give audiences a great bad guy and a sharp definition of Diana's personality -- which includes her hope, desire for peace, and preparedness for war.
Aside from the logic of using Ares in the first Wonder Woman movie, it's worth remembering that other classic Wonder Woman villains can always show up in the sequel. Bad guys (and girls) like Cheetah (possibly the only other baddie who can rival Ares in popularity), Doctor Psycho, and Giganta are all waiting in the wings now that Diana has received a proper introduction into the live-action DC landscape. Patty Jenkins is already gearing up to dive back into this world with Gal Gadot when Wonder Woman 2 hits theaters on December 13, 2019, so it's anyone's guess as to who Diana will face off against now that Ares has been dealt with.