Plenty of people have already said Voyagers, and in terms of the sheer step-down in quality of on-board experience they have to be a strong shout compared to the Mk. 2s they replaced. They are a quite frankly awful experience on anything but a short hop - cramped, smelly and almost always overcrowded, and have been since day 1. Some people excuse them given the supposedly unprecedented increase in XC ridership, but given it was already rising when they were procured, the lack of hindsight is equal parts frustrating and depressing - cross country rail travel is worse than it was 20 years ago thanks to their introduction.
Can't moan too much about Pacers because they kept the continued operation of a lot of rural routes feasible at a time when the Thatcher government was toying with a second round of route cuts to save money. Sure, they're crap on jointed track, but they do the job and still do 30 years later, which is actually quite impressive given they were always seen as a stop-gap.
That said, this particular accolade surely has to go to the one-off
Leyland hauled coaching stock, which was by all accounts an attempt to apply the same... ahem, "principles" as the Pacers to the ageing stock of the regional and intercity market. Luckily, it was rightly dismissed as the carbuncle it was and BR retained and refurbished existing Mk. 2s instead.
God forbid if they had actually become the standard for cross-country stock - I might be sat here praising what a revelation the Voyagers have been!
Edit - @Darandio beat me to it, but the point still stands! 