At least the 365's didn't suffer from the perma-irritant that was the cooling Fan in the 166 version. Most of the GW units have rather excessive and whining fans which sadly make travelling in that section rather off putting now.
On the Intercity front, I would have to rate it along the following lines:
Best - Has to be the GWR / ScotRail HST First Class. For the reasons outlined in the above comments, they really did set the benchmark high with those.
The worst - It has to be a toss up between VTEC and the IET. The VTEC refurbishment did the opposite to the GWR refurbishment, it splashed Virgin Red & Black everywhere. It made the Primarius seats have the appearance of an aftermarket boy racer car, while the rest of it felt well, lacking in something. As for the IET - need I say more? Especially on the GW front. From a Warm, Welcoming, Luxurious FC with warm muted lighting and colours to a Cold, Harshly lit, Dull colour palated, Rock Hard seats and all in all something frankly not worth the upgrade costs for.
On the suburban front, I agree with those mentioned the 377/6,7 & 387. I'm also not so keen on the relocated 450 FC either - it worked as it's own separate compartment, now it feels rather condensed and squashed in.