I love this thread.
My long and boring thoughts:
I think ALW has written some amazing things, in particular Evita. I think the music in Cats is brilliant as well. Jesus Christ Superstar is my childhood, and one of the first CDs I ever owned was The Best of Andrew Lloyd Webber.
That being said- nothing by ALW has never shocked me with its beauty, or complexity, or intricacy the way Sondheim does.
When I listen to On the Steps of the Palace, or Ever After, or Your Fault, or Sunday, or We Do Not Belong Together, or Not a Day Goes By, or The Miracle Song, or A Little Priest, or the last bit of Everybody Loves Louis (right before she shouts "Louis it is!") or Not Getting Married Today, etc. I am reminded of what music is, and why I love musical theatre.
I think one of the things that often makes Sondheim music doesn't even rhyme - it's just dialogue with notes (see We Do Not Belong Together), and then the other half it rhymes so much it's overwhelming (see On the Steps of the Palace).
Also, nothing will ever be as good as the lyric "It's your father's fault that the curse got placed and the place got cursed in the first place."
"Nice is different than good."