Blondie - One Way Or Another. Never actually released as a single in the UK but a hugely popular song that everyone knows.
The single 'Thriller' only made it to #10 in the UK, despite being one of Michael Jackson's biggest hits. 'Purple Rain' never made it to #1 in either the UK or the US, despite being Prince's signature song.
But the crowing glory imo is Madonna with 'Holiday' - an absolute favourite of the general public, but it ever went top 10 in the US and failed to get to #1 in the UK despite being released here on 3 seperate occasions.
Girls Aloud: Only 5 years until their 20th anniversary reunion tour! (and only 5 years until I can change my signature...)