Of those whose buses I've used at least semi-regularly in recent years:
The Very Good: Stagecoach Oxfordshire, Ensignbus, Stagecoach East London, Stagecoach Fife Scottish.
Of these Ensignbus have been consistently first-rate in each of their incarnations, in terms of quality of service, innovation, and good quality vehicles (new and very old). The Gold bits of Stagecoach Oxon outrank the rest of their services. SEL do what they have to do, and do it well. Fife Scottish likewise, with added innovation.
The Fairly Good: Oxford City, Stephensons of Essex, Metroline
The More or Less Acceptable But Could Try Harder: Thames Travel*, Transdev London, First Centrewest, Regal Busways
*they seem to be some way through a process of acquiring newer vehicles, so may well improve, even if it means that they have borrowed in some rather decrepit and ageing and step-entrance buses from other Go-Ahead companies in the meantime.
The Really Rather Poor: Arriva Southend, First Thamesway (nothing to choose between them as far as I can tell. Cost-cutting is all they have been about for some years. Maybe Arriva are marginally worse, though.)
The Atrocious:....they went bust last month so don't need to be named