I'd ask what kind of stuff they like in general and then go with some great classics within those genres. There is quite difference between shonen action comedy and romantic slice of life after all.
If i wasn't given genre, I'd recommend sth great that isn't too niche and includes lot of genres. Angel Beats! is kinda my go to since it's simply awesome, it's short so the lenght shouldn't be issue and it includes pretty much everything: action, comedy, drama and tears, school and romance, some life philosophy.. It's also really "anime" I mean it looks like anime and includes many anime tropes though not in bad way. So after that person would have general idea what anime is and if it would be sth they are interested to.