the sound of music seems to be the one that even today kids get into, I think as far as a musical that spans generations and creates interest even from children that is the one that comes to mind. I personally like the music man and the pajama game
West Side Story was considered one of the biggest gambles in its day because it was considered very controversial, due to the racial overtones, the murder on stage, and several other factors. It shocked the public, in fact, but obviously in the positive ways intended.
As a result, it is considered one of the "holy trinity" of musicals on Broadway, having influenced dozens of other shows to follow its lead. I don't know that it's the most influential, because "A Chorus Line" also revolutionized Broadway in a different way. There are also others that have changed the dynamic of stage musicals, but these are just a couple.
It would be hard to narrow down to one or two shows which would be considered the most timeless.
"The King and I" probably at least belongs on the list, but all of the Rogers and Hammerstein ones are great-- and since they are old "test of time" applies. Then there's always Gilbert and Sullivan-- and earlier operas like Aida, and Madame Butterfly.
I think one Musical that goes right back to the beginning and was probably one of the most popular was the one made with Julie Andrew called 'The Sound of Music" I don't think you can go past that, the music is still sung all over the world.