What Doors album would you recommend to someone if you could only pick one?
The Doors Greatest Hits 1980
It's not too long and it's perfect!
I would have to agree with queenhwy that a compilation album would be the best route, but if I interpret your question as to which single studio album, I would recommend their debut album, "The Doors." That album was cut in a little over a week, with a lot of the tunes done with one or two takes, and introduced the world to a group that was not only flying in the face of music convention at the time, but to a group that was to change the course of rock and roll like no other group since the Beatles.
Wow! Tough choice. I couldn't pick, so how about a best of album with a mix of songs through the years. This way they get a wide spectrum of tunes!