Sweeney Todd-This is Sondheim's magnum opus! Nothing compares to the dark complexities show this brings to the table. I am talking however about the original incarnation of the show and not the joke of a revival a few years ago.
I couldn't agree more with this post. Fellow Sondheim lover here.
I have seen, in no particular order:
Fiddler on the Roof - original production - the first show I ever saw when I was a little girl
Cats (hated it)
Sweeney Todd (original cast w/Len Cariou, Angela Lansbury, Victor Garber -

saw it three times!!)
Phantom of the Opera (didn't like it)
Once Upon a Mattress w/Sarah Jessica Parker (it was only OK)
Thoroughly Modern Millie (it was OK)
Oklahoma!

thumbsup2 revival about 5 years ago)
Gypsy w/Bernadette Peters

thumbsup2 loved it)
Gypsy w/Patti Lupone (hated Lupone and the girl who played older June, but the rest of the cast was OK)
Into the Woods (revival w/Vanessa Williams)
Beauty and the Beast (good)
Mary Poppins (good)
Cabaret (most recent revival - production was very good but we saw Brooke Shields who was just awful)
Good Vibrations (awful - ran for maybe a month)
Wicked (good, but I prefer the original cast album to the cast we saw)
Sunday in the Park with George

thumbsup2 last year's revival - one of the best things I've ever seen)
South Pacific

thumbsup2 at Lincoln Center - wonderful production & excellent male lead Paulo Szot)
I don't know if this counts but I just recently saw a concert reading of A Little Night Music with a great cast including Natasha Richardson, Victor Garber, Vanessa Redgrave and Christine Baranski - I wish that there could be a full revival of this show, it's wonderful.
Hoping to see the revival of West Side Story next.