I aren’t voting because I don’t think a group of rail enthusiasts can decide what is ‘best’, I always think threads like this should really be titled ‘favourite’.
However I shall look at the 3 trains individually and give my own opinions on each.
HST - It was a game changer in the 1970’s when it entered traffic. For all those complaining about the original seats (comfy enough for me but the armrests are a ballache which means they are poor by modern standards) they were a step change from dismal mk1 & early mk2 coaches. They were fast for the times and ride reasonably well (not as well as some people like to make out). The interior has been altered over the years but for me the best mk3 refurb wasn’t on an HST but the ATW refurb with those comfy grammar seats in the standard (not original) layout.
They were known as the journey shrinker and that’s exactly what they were when introduced. It was the best in it’s time but was soon dated hence the various mods over the years culminating in power operated doors for the coaches and re-engining etc.. for the power cars. Because me and HST are about the same age and I grew up with them from new I have always had a massive soft spot for them, especially the original ECML locos but time moves on.
Class 91/mk4 - I have always liked these trains and it’s such a shame that they were not allowed to show their full potential (tilt/140mph). The locos were troublesome (not the only locos to have issues from new) and the mk4’s needed bogie mods to improve the ride. However their performance was even better than the HST’s they replaced and they were the absolute stalwarts of the ECML for 30 years.
The coach interiors were dark and moody in FC and bright and modern with very uncomfortable seats in SC from new (at least they had moveable armrests though). The Mallard refurbishment was decent enough which was then improved by VTEC when they went for a comfier seat cushion in SC and the lovely mood lit FC. However the best interior refurbishment was done by GC. The FC looks every bit as classy as the last GWR FC refurb. Again I have a real soft spot for these trains as I saw them from development and was mightily impressed by the class 91’s noise and power as a teenager.
Class 390 - Ah yes, the marmite of trains. My opinion is somewhat limited on these trains but I have been on a few in both classes over the years. Firstly the elephant in the room, the windows. I can’t help thinking that I read somewhere that the windows were so small due to the 140mph planned top speed and crashworthiness but I may be wrong. I don’t actually mind the window size as I aren’t a window gazer particularly but I should think that a bit of planning for reserving a proper window seat should solve this issue. The seats I find very comfortable in both classes (I’m 183cm) and I do like the FC ambience.
The tilt is fantastic and you can just notice it enough so it doesn’t cause the nausea of the APT. They have totally revolutionised the WCML compared to the ageing locos and stock they replaced. Again, it’s a shame they couldn’t be utilised to their full potential with regards speed. Hopefully the refurbishment will improve on a decent enough train.
So that’s my opinion of the 3 and I don’t necessarily think anyone’s opinion would confirm which is best (as I say, favourite would be a better word). They’re all decent trains which improved the routes they worked on in my opinion with their own flaws (nothing is perfect).