For me, the best would either be HST (LNER/EMT) or Mk4 (LNER). Lovely comfortable seats, attentive service, and an extremely pleasant ambience for the journey. For shorter trips, I must also give an honourable mention to the sadly soon to be departed GN 365s - again, they have lovely comfortable seats - definitely nicer than those in Standard (although Standard in a 365 is significantly more comfortable than their GN/TL replacements’ offerings). The worst - without a doubt, the GN 387s. The location at the inner ends of the DMCOs is awkward and means that Standard passengers seated at the outer ends of these coaches have to walk through First to access the toilets... and when they do, they can’t fail to spot that the only discernible difference between Standard and First is that the seats in First have antimacassars!