Difficulty is one thing but what you are aiming for is for whoever evaluates it to say “yes this is one of the very best I heard this year.” Being in the “pretty good but as good as other people” category does not help the way you want. So play what you are best at in terms of interpretation not just technically. And I’d also say avoid old chestnuts unless you are super confident that your version will outshine.
Having said that, my daughter was admitted having submitted a vocal portfolio that included “O mio bambino caro,” which is a real warhorse piece that everyone does, so I am not sure she practiced what I’m preaching here. Her two other songs were operetta and broadway so her range was at least broad 
good luck