Historically, so many great seasons. Hard to beat the seasons that saw the original productions of WSS, The Music Man, Funny Girl, The Sound of Music, etc.
For me though personally:
1994---Carousel, Damn Yankees, Grease, Showboat, Sunset Boulevard (also saw Kiss of the Spider Woman and Miss Saigon from previous seasons)
1995---Hello, Dolly!, How to Succeed in Business Without Really Trying, Master Class, Smokey Joe's Cafe
2014---Aladdin, Beautiful, Bullets Over Broadway, On the Town, The Curious Incident, Lady Day
2015---An American in Paris, Fiddler on the Roof, Finding Neverland, Fun Home, Hamilton, On the Twentieth Century, Something Rotten!, The Color Purple, The King and I, The Visit
Fiddler in 2015 was super special because it was the first show I ever saw on Broadway when I was a pretty young kid in 1971. Seeing it again in 2015 was like traveling full circle.
I don’t know my theater history enough but I thought 2013-2014 was strong based on a binge theater weekend I did of R+H Cinderella, Pippin revival, and Kinky Boots...capped off with that spectacular “Bigger, We’re Goin’ Bigger” Tonys opening number.
2005 Best Musical Nominees: Spamalot, Light in the Piazza, Spelling Bee, Dirty Rotten Scoundrels.
That is quite the foursome for one season.
On the non-musical side: Virginia Woolf (Bill Irwin/Kathleen Turner); Doubt, Gem of the Ocean, Pillowman, On Golden Pond with James Earl Jones; and Gem of the Ocean by August Wilson.
2015 was my favorite year of the decade. And more so if I include Roundabout's Cabaret which closed on March 29, 2015 and featured Emma Stone as Sally through February 15th and Alan Cumming until the close.