My choices are the IEP and Cross-Country 170's and Voyagers. Cheap plastic interior, horrible window blinds and in the case of the IEP, uncomfortable seats that are anything but luxury 1st class.
Cross-Country 1st class is poor value for money (especially at Weekends) grossly overpriced for the experience and environment provided.
In the 1980's and 1990's First Class was much better, more comfortable on HST's, 225 sets and loco-hauled Mk 2 stock. GWR now have less 1st class capacity than ever before (HST's used to be 2.5 mk3 coaches, which was reduced to 1.5 mk3 coaches before they stepped down) - I think GWR and other operators have delibrately lowered the quality of the 1st class offering so that less people use it, giving the operators the perfect excuse to abolish the 1st class offering completely. I personally enjoy using 1st Class for longer journeys, as it offers a more relaxed environment away from screaming kids and drunken idiots and bad behaviour (most of the time), plus the seats tend to be bigger and recline with more leg space, meaning more space and comfort, ideal for long journeys. My worry is the constant debasing of 1st Class coupled with extortionate prices will see this option for longer journeys in comfort disappear and more people taking to their cars instead.