I often try to get my flat mates to listen to sound horizon when they're in my room xD
Since no one I know understands japanese at all, I always go for the ishidatami no akaki akuma PV with lyrics since Shaytan looks awesome plus the lyrics are very passionate.
Also, you have to choose something where people can't pick at it because people like just being nasty

Thanatos is the song that got me hooked on SH so I would say that one but Yoiyami no Uta has perhaps the best opening riff/verse i've ever heard in a song. The melody of the guitars and the girl singing in time to them is what made me fall in love with them.

I had one friend who doesn't know what is Sound Horizon and I show to her the complete "Märchen" concert. She was really excited and when I show her the "Elysion" she "exploded"! because she couldn't believe that Aramary could be so awesome! X3

She said that she loved Stardust and Yoiyami no Uta ♥ so... I think this two songs are my options and...Shiseru Monotachi no Monogatari (Ιζηορια) :3

It's so sad how it really depends on who are you showing something. One of my friends liked Hiiro No Fuusha, others were like "Ok........" And I heard just lately, that I had given accidentally Roman and Moira on a memory stick or what they are along some other stuff, and the one I gave them to had listened those and really liked them. Funn...

But umm, yea. Depending on the person: Hiiro no Fuusha, Lafrenze, Endia and the Knights, Asa to Yoru, Utsukushiki Mono, Shinwa.
And if the listener likes violin, absolutely Rein no Sekai (Live).

A friend of mine introduced me to SH, but the song that really got me hooked was Meiou, Thanatos. If i had to choose, i would use that song to introduce someone new as well.