Well, I've done this challenge for every region except sinnoh so far and every time, I never picked legendaries. Mostly because I think it's cheap and also, because they're not my favorites :p. And what's the point of starting a challenge if you're gonna cheat? I do it just for fun. I would hope I could trust people to not lie about their favorites. I also wouldn't tell my favorites, just in case someone wants be a jerk like in your example. And maybe there can be a legends version and a no legends version.