The older 377s (377 101-119), as well as the 377/3s, win it for me. The 2+2, very comfortable seating throughout these units really makes for a pleasant journey (I don't even mind being caught up in the delays Southern throw at us when travelling on these!).
377 120-139 are not too bad either, but they have 3+2 seating in the middle of each coach, with 2+2 at each end.
As for the younger 377s, again, some of the seating is comfortable (the 2+2 sections, located in the outer coaches), but if you're unlucky enough to be in one of the centre coaches, it's not a pleasant ride!