For me, I'd pick these 5.
1. Goodbye to Love. My all-time favorite Carpenters song, one that is gripping from start to finish, with a melancholy vocal from Karen and the fuzz guitar solo. A haunting juxtaposition that I never tire of, no matter how many times I've heard it.
2. Only Yesterday. A song that still sounds fresh, includes the best of Karen's lower and higher register, and the creative vocal arrangement and harmonies that made them successful. Also, a song that captures the best of the Carpenters' happy/peppy music, but still against the backdrop of the melancholy of 'yesterday.'
3. Rainy Days and Mondays. A riveting arrangement serves up Karen's mournful voice. I like the post 1991 remixes, where the strings behind the sax solo are eliminated, leaving a rawer sound.
4. Our Day Will Come. The 1997 remix (without the DJ interrupting) is my favorite. I love how versatile Karen is in lending her voice to this fun, jazzy song. Her voice is at its peak during 1973-75, in my opinion.
5. Let Me Be The One. The 1991 remix with outtake is my pick here. The bridge is heart-melting, with Karen in her warm, lower register, against the coldness of the piano and starkness of the arrangement. ("A loving touch" - and then the drop in the bass). It is one of the most understated and beautiful moments in all of their music.