A two-prong question, part memory and part opinion. For the purposes of the question, I’d generally rather not address greatest hits collections or best-ofs, just original issue recordings.
What’s the most effective use of a ballad as the opening track of an album, and what song could open an album? Many artists have opened their recordings with an uptempo song, what George Martin called a “potboiler” in reference to Beatles albums, but not as many do it with slow songs.
For the first part of the question, I’ll go with my fave: The Band’s “Tears Of Rage” from Music From Big Pink. I can’t imagine that album starting any other way. A chancy move in its time, starting with a slow song, but it really stood out as a result.
The one I think could open an album would be “Rocket Man” from Elton John’s Honky Chateau. I dubbed this song as the first one on an 8-track tape in my younger days, and to me, it still works as a great, if moody and dark, opener.