Since there are not as many non-American musicals to choose from (the "musical' has widely been deemed an American invention of sorts) And since the art form of the musical has evolved with each passing musical era, it's very hard to pinpoint THE greatest of them all. Perhaps it hasn't even been written yet.
West Side Story Gypsy Oklahoma Hair Rent Sunday In the Park or Sweeney Todd A Chorus Line
Many of these have left an indelible mark on the theater landscape be it subject matter, strength of book/music, the style presented being quintessential to the art form, the mad talents of the creator/composer, the radically difference in tone/music/storytelling that redefined the genre, etc.
One of the great "American" musicals in my opinion is "Ragtime" for not only it's historical perspective, but for the adherence to the thematic beats of ragtime music found throughout the score, giving it a real 'heartbeat' using a style of music that was very distinctively American.
It's very sweet of you to be so loyal to Ms. Halenda, but you will learn that there are degrees of stardom. To conflate a small star like Kathy Halenda with a large star like Angela Lansbury is sign of immaturity...or dementia.
I was once as deluded as you. When I was 12 years old, I was taken to see Mame on Broadway, then starring Jane Morgan, whom I instantly pronounced "better than Angela" (whom I had only heard on the cast album).
To give you an idea of my immaturity, here is Jane Morgan performing "It's Today." She is not better than Angela. I'll let you decide whether she is better than Kathy Halenda.
She is currently starring in a third rate touring production of GYPSY. She has never been on Broadway..which is probably why most haven't heard of her.