Now and again I come across album tracks, B-sides, etc. that I think could have made great singles if given the chance, such as the Talk Talk B-side "Again a Game...Again" or The Human League's "Things That Dreams Are Made Of". I often regard "White China" from the Lament album as being in this category and there are plenty of examples out there by other artists.
The track "Panopticon" by White Town is a driving synthpop gem and yet it only appears on the 2003 Robopop CD compilation, nowhere else (https://www.youtube.com/watch?v=x6jLhZO9CBg
). And surely the Pet Shop Boys' "In the Night" was a potential top 10 hit? In the liner notes to the PSB compilation Alternative, Chris Lowe relates how the cutting engineer was convinced it was the A-side and when they explained that it wasn't he thought they must be mad.
The third movement of Jarre's Magnetic Fields 1 had potential too, more so I think than "Part 4" which was the one actually released (after the earlier "Part 2" had been a hit). It's the section that begins with the sound of a jet plane flying past and is quite a catchy, driving tune. It did eventually get a 7" release in the Netherlands but only as a b-side:- (https://www.youtube.com/watch?v=UXWQIRL7sGo
I'd be interested to hear about any other such songs that are worth seeking out.