In my opinion I would say, of the first class trains that I have used they would be ranked from worst to best in the following order:
Southeastern class 375
Thameslink class 700
South Western Railway class 450
London North Western Railway class 350
East Midlands Trains 222
Great Western Railway class 800
Cross Country class 170
Great Western Railway class 166
South Western Railway class 159
Cross Country class 220/221
Greater Anglia Mk3
Cross Country Mk3
Virgin Trains class 390
Trans Pennine Express class 185
Virgin Trains class 221
GWR (now Scot Rail I think)/LNER Mk3