It's not my absolute favourite, but it's hard to deny it's got a brilliantly inventive but also completely accessible score and choreography, with Sondheim lyrics, and I don't think there's a dud song in it. Even the ones I don't personally care for are incredibly well-crafted (and incredibly challenging to sing), and it's got pathos and tragedy and soulful solos and duets and mass dance-offs and teenage aggression, and murder and silliness and ethnic gang hatred and meditations on being an immigrant, but it's also very, very funny -- I love 'America' and 'Gee, Officer Krupke'.
I find Les Mes a bit ponderous (hardly surprising, given the subject matter), Evita I don't really know, Cabaret I like, but it's a bit thin on songs, and there are an awful lot of reprises, plus you're stuck with the fact that Sally Bowles is supposed to be crap! I'm never sure it stands up as a stage musical, rather than the Liza Minnelli film version.
I adore High Society, but in some ways, that's barely a musical for me, or at least not in the way WSS is. You could take out the musical numbers and still have a coherent film, though I do love 'Who Wants to be a Millionaire?' (Celeste Holm is so underrated) and 'Did you Evah?' Also, Bing Crosby looks like he's sleep-walking. When they did a Broadway stage version in the 1990s, they had to put in a load of other Cole Porter songs from elsewhere to bulk out the music.