When I applied to YPN, and got accepted, I used the URL with the most traffic in my portfolio. It was a wallpaper site getting quite a few million page views a month.
Same with me, I used the url with the most traffic as well. like 100,000 uv a month. I pretty much thought thats what they wanted. I just got accepted and I applied about a month ago. I also run Adsense on the site so I think that makes it look less risky because I'm trusted by google and obviously following the rules. The site was a niche health site.