Follies is known for its innovation, lavish sets and costumes, classic score, and the whopping original cast. It is something of an academic classic, celebrated among those who are theatre savvy rather than known to the general public, with the exception of a few songs (not the the general public would know what they were from). Unfortunately, the show never found real success in fully staged production (it's the book, if you ask me) and no one has really agreed on how it can be fixed.
Two Gentlemen of Verona was fluffy entertaining piece that was cleverly constructed and ingeniously staged for one purpose: to delight and enchant the audience. It succeeded and won the top prize while Follies won the Tonys it deserved, in my opinion. The biggest distraction in its writing are the clunky lyrics, but I think it is one of the most underrated musicals out there. I saw the production at the Delacorte and while it needed some comedic tightening and the choreography could have been beefed up a bit, I instantly saw how it won Best Musical and the dated style of the piece actually worked in its favor. I don't know why it didn't get a Broadway transfer (Norm Lewis and Renee Elyse Goldsberry would have been major Tony contenders). Even with its faults, it still remains one of my favorite shows I've seen in New York ("Bring All the Boys Back Home" was entirely relevant in the Park and the audience cheered it wildly) and if I had to choose Follies or Two Gentlemen to see again, I'd have to pick the latter.
The Music Man is a classic and actually one of the most perfect musicals ever written.
West Side Story vs Hair? Well, it has nothing to do with the shows themselves so much as the staging and performances, since they are revivals. I get that you're trying to do something of a six degrees thing here, but it doesn't really work. Hair is quite different from Two Gents, much like Passion is quite different from Forum. And comparing revivals to original productions isn't quite the same either. Both West Side Story and Hair lost the Best Musical Tony their respective years to The Music Man and 1776. I think it is a little more interesting to see how they will duke it out the same year for Best Revival, though I'm 90% sure Hair will win.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian