Definitely MK3s. Having used Pendos for a couple of years regularly when they first replaced the MK3s, there was a very noticeable decline in comfort. Once you got past the modern exterior, internally they were/are just Ryanair on wheels. First Class is better but should be more spacious, the seats are not much wider than those found in standard class on other IC routes. Airline seating legroom isn't that great in First either. Voyagers have the same issues, except the windows are bigger which alot of people prefer. But they can be noisy, which you don't want on long distances! I don't really hate the pendos now as much as i used to, actually i quite enjoy a short journey on one. But theyll never beat a MK3 or MK4 for comfort. The missing armrest on window seats, what a joke! I like all the refreshed MK3s, favouring Chiltern and FGW's.
Mk2s from a long distance point of view i never liked, they were daughty and just as noisy as Voyagers are! The only good points were the seating and the legroom, that's it! Would probably choose a Pendo despite their interiors, at least they are quieter!
Those aside, really like 175/180s which have great interiors and are not too noisy. Meridians are sometimes nice, other times turbulent. Class 442s are great unless you happen to be in the centre car, not too fussed about the new seats as they only run to Brighton anyway. 444s are almost spot on, but they should've had better padded seats like FGW HSTs.
Got confidence in the IEPs, the fact that in the spec they Openly admit the faliures in the Voyager/Pendos is a good sign. Probably will be like an even better version of the 180...i hope so anyway.