I've seen some here argue that a good livery should be able to match the build of the trains they're applied to, but I would just like to personally argue further that a good livery would also be flexible enough to be adapted to different stock without much need for change. It's easy to say for example that the old Virgin livery was good because it suited the build of the Pendolinos, but Virgin had the benefit of operating a small streamlined fleet of intercity trains. But if you were trying to brand a franchise such as Great Western Railway which operates both regional and intercity trains you have a bit more constraints. You need a livery that matches all the stock and that's why I think GWR's livery is so good because of it being appropriately applicable to a variety of rolling stock.
If you're going for a variety of liveries based on different services like South West Trains did then you'd have a lot more room to play with, but I still think some trains are better suited to certain liveries than others. I certainly think the Class 444 wears the white Stagecoach livery better than the 442 and 158/159's did. That's not to say they're bad, I just think the 444s wore them better. CrossCountry does a decent job of applying a single livery to the Turbostars, HST and Voyagers, but again, it's kind of easier to work with a less varied fleet. I'm open to further discussion about this of course but I think a certain degree of brand unity is good for a train operating company, at least in some situations.