One callsign that surprises me is BOEING, particularly in 2016 there was a 787 heading to Farnborough with BOE787 as the callsign. There were several hundred in service by that time and calling 'BOEING 787 contact Scottish on 129.1' seemed to have potential for much confusion.
Airbus have AIRBUS INDUSTRIE which is marginally better, but why not something catchier and less confusable? EUROPLANE or something.