Renault - Had a Megane that had so many problems. Renault tried to charge me for well established manufacturing defaults. Sold it to WBAC and saw it was scrapped a year later.
o2 - Tried to double charge the last month on a contract exit, threatened me with legal action until I proved to them there are 12 months in a year, not 13.
Scottish Power - Refused to take dated GPS tagged photos of meter reading when I moved in, insisted on using massively inflated estimates. Awful customer service hold music.