Are multiple responses acceptable? As one or two have already put multiple responses.
I'm not voting myself as there is no "best" TOC. Some are worse than others, but none stand out above all the rest. For example, describing some of my local TOCs:
London Midland offer a punctual and reliable service with some very cheap Off-Peak fares, especially during their sales. Their medium distance rolling stock is more than acceptable, even on 3h+ journeys considering the price I am paying!
Arriva Trains Wales offer a reasonably good service. The recent station improvements are nice, but reliability can sometimes be hit and miss, especially with the infrequent service at some stations. Fares are good for medium distance journeys, but not short distance journeys. The rolling stock could also be better on many routes. Very nice rolling stock operates some of the services on one of my local lines, but only because they operate long-distance services which happen to be used by commuters.
Virgin Trains has nice, fast, frequent trains. They have good on board facilities, as well as good station facilities for First Class passengers. Their Customer Relations department is one of the best around. They have some nice "perks" (Railcard easement, free First Class upgrade at weekends for Anytime tickets), and have some excellent value Advance fares if you book far in advance. But their walk-up fares are sky high, which in my view prevents them from being the "best" TOC.