I would have to say "No Reason to cry" only because that is when I started to listen to Freestyle, and that is my earliest memory of the freestyle experience..
I would have to say TKA's "MARIA" becuase it was released when freestyle was on its last leg. Its a song everyone knows and remembers fondly. it can always stand the test of time.
Totally agree with Jason... Heartbrake - "This Is The Night" does it for me...
But the song that did take off in the UK (if only slightly) was Safire - "Let Me Be The One" which spawned a track by Tim Simenon of Bomb The Bass feat Lorraine - "Don't Make Me Wait" reached number 2 or 3 in the UK national charts in 1987...
My 3 top choices are Boy Ive Been Told by SaFire, Without You by George Lamond, and Tears May Fall by TKA...the opening bars to all 3 songs definitely DEFINE freestyle.