I usually can't sell them with just one song, but I'll make a mix CD. And the selection of songs depends on the friend, and what type of music I know they like and will appreciate. But quite often, a mix will include "Hey Jude," "Let It Be," "Golden Slumbers/Carry That Weight/The End," "Blackbird," "In My Life," "Yesterday," "Here Comes the Sun," a version of "While My Guitar Gently Weeps" (either the White Album version or the LOVE version), "Strawberry Fields Forever" (either the standard version, or Take 1 from Anthology 1), "A Day in the Life," and "Across the Universe."
I usually try to make a mix of both songs they will recognize (most of the ones mentioned minus the Abbey Road medley), and songs that they probably haven't heard but are great ("Dear Prudence," "Two of Us," "Real Love," "Everybody's Got Something to Hide Except Me and My Monkey," "Lovely Rita," "Good Morning Good Morning," "Cry Baby Cry," "Here, There and Everywhere," "For No One").
Even though Abbey Road is my favourite album, I find myself recommending The White Album, as I find that showcases a lot of diversity, as there are some very casual listeners who think that all Beatles songs sound the same (apparently the Beatles stopped recording music after 1964). But if my friend is a very casual music listener (i.e. will not listen to anything that isn't in the current Top 40), or doesn't listen to much music at all, it can be hard to convince them to go out and buy one of their albums. So sometimes I feel like I have to force the Beatles into their life.
It also never hurts to throw in a solo song or two ("Jealous Guy" and "Maybe I'm Amazed" will always make the cut when I include solo material)
