I've travelled in First Class on most long-distance operators that provide it, and also several shorter ones.
Among those I've been on, I think East Coast probably win, although they do have the unfair advantage of being my longest First Class run to date, which also probably won't be beaten any time soon because said journey is
the longest non-XC run in the country. An honourable mention goes to CrossCountry, on which the step up to First on Voyagers is massive and even now the pricing on the Central Belt is unusually cheap, if not as cheap as the £1.70 when I did this route
on an HST); a second goes to SWT whose provision on 458s is really quite remarkable for a suburban train, but then they have to compete with FGW on the Waterloo to Reading corridor so the effort needs to be better - and on my most recent experience, it was on a Standard ticket. A very special mention goes East Midlands Trains' provision on 222s, but that was more because I had enough tea to pay for pretty much the whole Advance from Sheffield to London - and that the "trolley dollies" were definitely the most attractice I've ever been served by on the railway
At the other end of the scale, the worst I have used, both for long-distance and short-distance, is by quite some margin Greater Anglia, although to be fair I am yet to use First Class on GA routes in the GA era. The First Class provision on their 321s is without a doubt the worst First Class I have used, and this is worsened by the poor attitudes of staff I have experienced whenever sitting in First Class (one told me my 1ST Advance ticket wasn't valid in First Class), and that available on the MkIIIs to Norwich is - or at least was in 2010 - nothing special either, with no complimentary items and no power sockets. I hope that this has improved in the GA era! A less than excellent mention also goes to London Midland and their 350s, but this is more because of (a) the location of their First Class facility and (b) the fact that I once cut my hand on the reclining mechanism...