Hellsing Ultimate is the best show to show to young to middle aged people. It has a brilliant dub, so they won't be sitting there unable to take it seriously because of Japanese. It has a unique, not-very-anime artstyle that doesn't seem to be any particular nationality. It's bloody and quite dark, so they won't underestimate it as some weird Japanese light-hearted crap. And finally, it has Alucard. Need I say more?
Cowboy Beboop is old and, while probably very good, wouldn't be able to really catch a newbie who isn't into anime's attention
Baccano is downright bad as a gateway anime because it's so damn confusing.
Steins;Gate is very anime, so the newbie's prejudice come into play and makes it hard for them to get into it.
Gurren Lagann is good, but still not perfect.
Can't judge Psycho Pass since I've never seen anything about it. I would imagine that it'll still suffer from prejudice, though. (I've seen bits and pieces of Beboop, btw.)
Spice and Wolf is too... boring. Nothing really happens and it's a character driven show. Not very exciting, and thus wouldn't be able to grip a newbie very well.
Toradora is a highs school romance. Worst possible kind of choice.
Monster is only good if the newbie is a hardcore crime fan.
Shiki could be good, but very slow start.
Monogatari suffers from the same problem as Toradora.
Nah, I'm still the senpai.