I had forgotten about 45 years by Stan Rogers.

I also like "The Broad Majestic Shannon", by that decidely unsentimental band, The Pogues.

I suppose "Big Yellow Taxi", by Joni Mitchell.

I like "Highland Mary", by Burns. Although I have know it has a tune to it, I've never heard it sung.

How about "Ah, Sunflower!" by William Blake? Others have put different music to it, as Blake's own music has been lost as he lacked the skill to make notation of it.