HST, simply the best thing BR ever produced (although I declare an interest as a member of the 125 Group). Revolutionised passenger operations overnight soon after introduction. Has often been equalled, but never surpassed in this country. Maybe the TGV Sud-Est did the same in France, or the Shinkansen in Japan.
In Europe, the most comfortable train I've ever travelled on was an ICE1. High speed rail the way it ought to be, part of the network. It also had 3-a-side compartments in second class, was ice-smooth and had seats almost as good as a MkI.
Best current British loco would probably be the Class 60, simply because the things look like bouncers. Tough, solid and you don't mess with them. They'll shift anything.
Best loco of all time... A4 for passenger services (a bit unreliable, but the records speak for themselves) and 9F for goods. Maybe modern locos and units are better, but for the time, they were the very best that ever rolled out of a British workshop.
As for the worst... Well, it looks like a bus and it bends. However, Dellas are pretty foul. Apart from having seats like planks, there is virtually no air movement inside. I've sometimes spent two hours almost stifled inside one of them. They're the exact opposite of the ICEs, the worst of German engineering.