Well if you go old school with it, I'd avoid Sailor Moon, being in the fandom and whatnot, it's not only brutal, but totally overdone ... like to the point to where it doesn't matter how good your costume is, no one will look at it cuz they're tired of seeing SM cosplay XD (ok, exaggerating a little, lol). If you do decide to go with something Sailor Moon, do something that isn't done a lot, like villains or rare forms of one of the senshi. And sadly DBZ doesn't have a lot of girls, much less with cool costumes ... I cosplayed ChiChi and felt like I was just walking around in regular clothes, haha.
I'd just go with your favorite, like right now I'm making a cosplay from a video game that just came out a few months ago from Japan, I doubt anyone even really knows about it yet cuz it hasn't had a chance to blow up yet, but I'm still doing it ... every time I post a picture or something, I keep getting asked what it's from, but it's still cool, because that means you're doing something different!