I think I would pick Step You. I like Greatful Days, but a lot of people dislike it, and you don't want people to get a bad impression of Ayu. Moments is nice too, but I think it might be hard to sing for a guy. There are some rather high notes in the chorus.. But I don't know your voice, maybe you can do it
Heaven is nice too, but it's so sad.. A cheerful uptempo song like Step You would be great. Hopefully they'll want to get up and dance when they hear it
Anyway, good luck with the contest, I'll keep my fingers crossed for you!