I would have to go for any 22x.
Narrow trains due to the tilt profile meaning limited overhead storage and a cramped feel, smelly toilets, inadequate luggage space, loud vibrating underfloor engines. To think that these were a successor to MK3 coaches makes me sad every time I use them.
They are also in a petty poor condition now with the 222s standar class being especially poor for an Intercity service to London. Whilst they are also often inadequate for the services operated with people standing for long periods .
A decent refurb and some new stock would solve the last two points but even with this I will never love the 22x trains and would not mourn their passing to Booths.
I would go further and narrow it down to the Class 220. The Class 221 is slightly redeemed by its capacity; the class 222 more so as it took on some lessons learned from its older siblings.
So yes, I present to the forum
the Class 220...
Let us consider this train. Intended to function as the main workhorse of the cross-country network from Aberdeen to Penzance covering the large majority of our nation's great cities. Yet it has a grand total of 200 seats. Fewer than a 3-car class 158.
It drinks fuel.
It whirrs. It buzzes. It beeps. No one knows why.
It has windows. Somewhere.
The seats were designed by an alien with only a rudimentary understanding of the human anatomy.
An alien who also thought that toilets were very, very important. So important that a large amount of space should be devoted to them, and that everyone in the train should know what they smell like. All. The. Time.
There is nowhere to put luggage thicker than a ring binder.
And by all accounts they are incredibly expensive to run.
Other trains may be older, even less comfortable, but they aren't designed for passengers to be in for several hours at a time.
All in all, after a few hours on one I can't help but think of Agent Smith's speech in The Matrix:
I hate this place. This zoo. This prison. This reality, whatever you want to call it, I can't stand it any longer. It's the smell, if there is such a thing. I feel saturated by it. I can taste its stink, and every time I do I fear that I have somehow been infected by it. It's repulsive! Isn't it? I must get out of here. I must get free.