Speaking of the most familiar piano song, the first one jumps into my mind is "Für Elise" (For Alice) by Ludwig van Beethoven. This great German musician composed this song in early 19th century and it has been popular for over 200 years. Basically everyone around the world is familiar with its tune. Every time I listen to this piece, I can feel the sense of romance flows through those notes and make me calm down.
Since "Für Elise" is a piano piece, I got no chance to practice this one when I learned the violin. My teacher always assigned me to practice songs for violin specifically. Fortunately, I have a grandma who is very passionate about music and started to learn the piano when she was 65 years old. ( I admire her very much) She practiced this song a lot. When she played the piano, I liked to sit behind her and watched her playing. She was always so absorbed that I didn't want to interrupt her. I just stayed there quite and listened. I enjoyed that.
This is the first time I try to play "Für Elise" with string, especially with Ukulele. This is the special version for Ukulele and it simplify many parts for beginning players. Hope you are able to feel the very little part of beauty of this song through my "performance". I also want to dedicate this song to my beloved grandmother.