The worst journey I've ever had was actually on a 377. Now I must add that in its defense, it was an abnormally bad experience, and the unit in question didn't seem to be very healthy (we broke down at Southbourne, Crawley and Gatwick), even though I'm really not a fan of Electrostars generally.
Was on a Southampton - Victoria service, and coming up through the Arun valley was... exciting. The thing was lurching and banging all over the place, to the point that things were falling off the tables. You couldn't stand up it was shaking around so much. Didn't help that after the first failure at Southbourne the aircon went down and never came back. We started pulling away, there was a clonk, it stopped taking power, the lights flickered and the PIS reset. Happened a couple of times until it managed to get going.
377s seem to ride particularly roughly through that stretch (Arundel through Crawley), but this was something else. Also once we got onto the fast after Gatwick, it seemed to be trying its hardest to shake itself apart getting up to 90. There was a horrible rattling noise coming from somewhere, and it seemed to accelerate in bursts, followed by some juddering, then another surge of speed. Definitely wasn't a healthy unit. At least it made it back to Victoria.
Interior wise, I can't stand anything with IC70 seats, since they make my back ache - I'd genuinely prefer an ironing board. The Great Northern 317s were interesting, where if you leaned forwards in just the wrong way, half the time the seat cushion would fall off the frame.
The worst overall class of train I've been on for me would probably have to be a tossup between 317s and Southern's 455s.
