I have to agree with the Ibis, Luton - a dungeon, and with the Britannia, Manchester (both the one in the centre of town and the one in Stockport - both DIRE).
To these I would like to add the Ibis, Toulouse and:-
The Worst Hotel Ever, the St. James in Grimsby.
Fights between winos outside (and sometimes inside), grafitti in the lifts, dirty bedlinen, strange growths on the shower curtains, saucers full of cigarette butts under the bed...