It took me... let's see...1998...13 years to find this.
13 years.
In all Seriousness though I think the longest one to sit in my head from a child was PM Dawn - Set Adrift on Memories Bliss (1991), I only started hearing that song in 1999 (then all of the epic tracks that I would so very much have enjoyed went off air, and then it was like BACKSTREET BOIZZZ 'N COUNTRY SHIET)... or maybe I think it is because I probably remembered the FAMILIAR sample from Spandau Ballet WHICH WAS IN 1988
Enjoy The Silence - Depeche Mode - (WTC Tribute !!!) (http://www.youtube.com/watch?v=1rdfFroO67g#)