I'm going to try to be as objective as possible here, but that's still going to prove to be insanely difficult. I've only heard a very limited amount of the particular songs the world has to offer, so I can only personally offer input through my own limited experience. The margin for human error is is alarmingly high. With that said, here are a few which I would consider appropos for such a compilation.
1.)
Ludwig van Beethoven - Für Elise
My first reasons for its inclusion are its long term popularity, coupled with the simplicity of its being played on a lone piano. The melody itself is lovely, haunting, and completely non-threatening. It's extremely melodic, and I'll wager that 90% of the people you meet will be familiar with the song, if not able to identify it outright. As such, the impression it's made on humans is fairly undeniable. There must be something especially appealing about it. I definitely feel that its being included would prove a good selection.
2.)
Queen - Bohemian Rhapsody
This particular piece enjoyed very widespread (and rather longterm) appeal. It's compositionally interesting, with some gorgeous melodies and harmonies and pleasing vocal delivery. While it stands to reason that aliens with have different concepts of what is pleasing as far as sounds go, this does appeal to a widespread human audience. It's not especially simplistic, but it seems that structurally, it would appeal to any lifeform with any manner of mathematical knowledge or appreciation.
I may add more later; everytime I try to think of a new one, too many arguments against its inclusion arise in my mind.