From personal experience id likely say Thameslink is one of the Worst TOCs at least within the Southeast, extremely overcrowded trains especially on the Brighton Mainline at Weekends, frequent cancellations, big issues with antisocial behaviour on their "metro routes" Rainham, Sevenoaks, Sutton, not helped by being entirely DOO, units covered in graffiti, windows etched and toilets covered from floor to ceiling in graffiti. Not a pleasant user experience.
And as for being a 24/7 operator for Luton and Gatwick Airports, they are a disgrace, frequent cancellations of their overnight services, depended upon by airport workers and travellers alike, in some cases i have seen almost all the overnight Bedford to Three bridges services cancelled. Personally i now never rely on Thameslink for getting to Luton Aiport, whilst for early morning flights at Gatwick they are often the only option now, with national express being super infrequent. The axing of overnight services from Victoria with Southern and Gatwick Express is much lamented, as they could actually be relied upon to run, unlike Thameslink. And thats without mentioning the extortionate fares charged by Thameslink for journeys to Luton. And yes i am aware technically the TOC is GTR encompassing TL, SN, and GN, but i have little reason to complain about Southern and hardly use GN, hence me specifically picking out Thameslink
By contrast Greater Anglia with its modern fleet particularly the flirts which are a huge uplift from a singular 153 on the rural routes, and even the 745s being a suitable replacement for mk3s, are probably one of the best. Sure the 720s are rather poor quality and the aisles so narrow its practically a hazard. In my experience GA usually try to pull out all the stops to make sure early and late Stansted Express services run, even if said services have to be diverted or run significantly late.
Meanwhile Chiltern have had a huge fall from grace with their 2023 cuts being particularly severe. Oxford services relegated to 165s almost exclusively, Aylesbury via Wycombe services reduced to a shuttle to and from Princes Risborough. Birmingham services cut back in many cases to hourly with no more Kidderminster, and many services to and from Stourbridge depot running ECS vice passenger. Stratford Upon Avon being reduced to a Leamington Spa shuttle all day, excluding the last train to Banbury. And thats before we discuss their outer london stations who have long been in need of a metro service of at least 2tph which even when Chiltern had the 172s never materialised. But now the likes of South Ruislip (SB) and West Ruislip (NB) and Denham Golf Club have been reduced to 2 hourly off peak, with the removal of the Gerrards Cross stopper. Its quite frankly ridiculous and is a mockery of a timetable, a 2hourly service within greater london is essentially useless. Can only hope for electrificstion of the Chiltern route from Marylebone to at least Aylesbury via Amersham and Wycombe in the future to turn the tide of what seems to be a managed decline