My suggestion as others have also put it, the Class 158/159. Not as reliable from the start as the 156 but they feel like a godsend on longer distance routes, they really have a proper feeling of comfort to them. Properly refurbished as on GWR, they feel more modern than the 166s and 165s that replace them. They also have end gangways to make them easier for trolley services which is a nice thing on these longer distance journeys.
For EMU I'm going to say the Electrostar family have been a great new train and an aesthetically pleasing design, with in my experience reliable air conditioning for the heatwaves we get each year now.
Locos I'll put forward the 59s as they have been hauling some of our heaviest trains up and down from Merehead and Whatley quarries for 30 years now.
Intercity services? I do love an HST like most people but the gangway creaking and that unsettling smell of the brakes does dampen it a bit, but with the 390s they have been hammered for two decades in considerable comfort with an on-board shop and a calm atmosphere.
How about the various Networker classes?
The electric ones? I do like those, but I just cannot say that the diesel variants are any good whatsoever. The Chiltern ones may be different but on GWR land they're slower than the 150s, the air con on the 166s makes a really irritating whine and even the one or two they've refurbished into GWR inside still look like they are falling apart!
