Hmmm..... since Scandal has such a different range for their songs, it would have to depend on the person you wanna show it too.
For people who love rock, I would go for Shunkan Sentimental or Nanka Buttobase. Nanka Buttobase has that awesome Mami solo that is just simply amazing! The live of this that was posted in a couple of posts ago would be good to introduce them with. They are totally rocking it and Haruna is just too gorgeous to be missed!
For people who love the calm music, I would go for Secret Base, even though they didn't compose it or write it. It's an amazing song to begin with, but with Scandal covering it, it was just beyond amazing. Also show them the pv because it shows their really close friendship and all those beautiful smiles<3 Just watch the pv yourself and see what I mean: Namida No Regret isn't also a bad choice as well.
For people who love easy-going music, I would probably go for Taiyou to Kimi Ga Egaku Story, Harukaze and definitely Love Survive. Taiyou to Kimi Ga Egaku Story's pv is really colourful and personally it's one of my favourite songs as well. Harukaze has a catchy chorus and in Love Survive, Mami's opening solo hooked me to the song.