I'm a very big fan. "America" has always been my favorite. You people should check out the Yes cover. This song is a close second though: https://www.youtube.com/watch?v=HwX0ej2wwzg
If you like harmony, I'd recommend Crosby, Stills, Nash, & Young (or one of the many sub-combinationation thereof) or the Mamas & the Papas. Listening to Mama Cass' distinct voice and the beautiful harmonies, it's hard to believe my parents could have called this "noise" at the time. I've got CD's of both CSN&Y and the Mamas & the Papas and found myseld listening to them endlessly (like weeks at a time - unheard of for me).
The song America makes me cry. I absolutely adore Simon & Garfunkel and pretty much all of Paul's solo work. Probably the best songwriter the USA has produced. Though I love Phil Ochs...
I love their music. You really can't find harmonies like that anymore... 'Scarborough Fair' and 'Sounds of Silence' are some of the most beautiful songs I've ever heard.