The best trains to ever run in my opinion, are based on ride,ambience and performance, here are my opinions;
The best overall train would be HST and loco hauled MK3’s, as they were very comfortable and rode well, and just great trains, and pretty much saved BR when they came out, revolutionised train travel with their 125mph top speed.
The 225’s when they came out, as they were a step change on the ECML, with power doors, improved ambience and speed, as they were able to do 140 mph, which the drivers of the time, though un official, definitely took advantage of the 140 mph capability when Running late etc, especially on the 3hr 59 min London-Edinburgh Scottish Pullmans, I had some very fast runs on these services.
These are controversial, but I’d say the 390 Pendolino’s are worth a shout, as they were a step change in speed and performance on the WCML, with their tilting capability and 125mph speed, transforming this main route up to the top league along with ECML, Though not good as HST for ambience etc, with their more cramped interior due to tilting profile, and ride from tilting, though this never bothered me and like the tilting experience.
I think from recent trains I have rode on, the best have been the Stadler Flirts, as they perform very well and truly fly, and have decent ambience and seem really well built and put together, but being Basher from 80’s and 90’s I’ll always think best trains were 37’s, Deltics etc, as just a great sounding Loco and Mk1 or MK 2 coaching stock just knocks any modern bland and boring train out of the water for me.