Night Riviera for me.
The MkII cars on the Caledonian with the rough fabric seats are horrible to attempt to sleep on or even sit on for 8 hours.
Agreed and agreed. I really best not get discussing the Cally Insomny here, I could be here for a while...
Most comfortable train for a long journey? I'm quite happy with FGW's HSTs even in Standard Class. Away from the Western, I'm happy with the 390 seats although I prefer First Class on those. Heading east, the MK3s on the GEML do a good job.
Definitely going against the grain here though, the 170s have good seating for a long journey in my mind. Well, if you consider New Street to Hereford as a long journey, which in my neck of the woods it is.
Abroad, the Irish Rail 22000 Class seats are pretty decent for the long journeys they do. I was perfectly satisified with them for a Heuston to Westport run at least, and the class do other long runs like Heuston to Galway (which I was also satisfied with), to Tralee and Cork. I preferred the 2700 Class for noise, as well as the 29000 Class for noise, but that's a bit off-topic...
Elsewhere in Europe, the Netherlands to be more exact, the ICMm EMUs are pretty comfortable and they do some rather long journeys. The VIRMs over there aren't too bad either. The coaches on the Liege to Luxembourg route are rather comfy too, and for modern double deck stock in Belgium I was happy with the seats I had for the Bruxxelles Midi to Chaerloi journey I made in 2013, said stock is also used for much longer journeys IIRC.