I’d say Gryffindor, but it’s entirely up to you :)
If your OC is going to be a direct descendant, will they have similar abilities as their ancestor? If you decide to go with Gryffindor, would you OC inherit duelling skills, for example? The sword of Gryffindor that Neville pulls out of the hat, maybe your OC could be a natural at sword fighting or something, and teach Neville how to fight using it (I think he uses it in Order Of The Phoenix as well as the last film?)?
Also, imagine the tension between the heir of Salazar Slytherin and the heir of Gryffindor (if that’s who you choose your OC to be the descendant of) in Chamber Of Secrets if you decide to go further with your story ^_^