Best:
"Oh What A Beautiful Mornin'" which changed the concept of the way opening numbers are supposed to be. Instead of a chorus singing with some big flashy number Aunt Eller steps out and churns butter while Curly sings. Amazing for its time!
"Overture" from Jesus Christ Superstar. It doesn't attempt to play highlights of the score, it's totally new, eventually becoming the music for "Trial Before Pilate".
"Requiem For Evita". This is absolutely stunning, IMO.
Most, if not all of Sondheim (I particularly like "Forum", "Sweeney Todd" and "Into The Woods")
"Denton U.S.A." from Shock Treatment. Well, not a stage musical but I think it's the best part in the movie.
"Little Shop Of Horrors", perfect! "Skid Row" is another great song
"Merano" from Chess. This should have been kept as the opening number. To me, the only "Chess" is the '84 concept album or something like it. I hate all the re-writes
"At The End Of The Day", the opening number in the original Paris version of "Les Miserables".
"Let's Have Lunch" from Sunset Boulevard. I love the whole counterpoint thing, plus it's an extended opening going on through a procession of scenes.
"Overture / He Ho Professor" from Tanz Der Vampire. The atmosphere was completely ruined on Broadway because the Overture is scored for a blizzard. It's not supposed to be a random overture opening in a forest in Lower Belabart.. whatever. I also like the extension of the Overture as Alfred searches frantically for the Professor.
"Gotham City / The Graveyard Shift" from Batman. I think this is as stunning as the opening for Evita. Although, of course, I just HAVE to mention Batman
Many more...
Worst:
I can't stand "The Producers" opening night song.
Who can explain it, who can tell you why?
Fools give you reasons, wise men never try
-South Pacific