While I'm happy to agree with EcclesCake that Brussels is fairly boring, I certainly couldn't consider it for the title of my 'least favourite city'. There are too many good bars, serving great Belgian beers, for it to hold that title ;-)
Pa___ul3 suggests Manchester. While it lacks the character of the two cities to the east and west of it (Sheffield and Liverpool), it's still got a few things to commend it.
I'm totally in disagreement with The-Toff-.
While Paris (like most tourist destinations) has plenty of 'rip off' bars and restaurants, you can still buy a good quality three course meal in the Latin Quarter for less than you'd pay for a rubbish meal in a UK fast food restaurant.
My least favourite city (with apologies to Barmaid who has the misfortune to live there) might well be Peterborough: Boring, boring, boring!
Chris