Worst regional variant? Two words - First Bristol. Never have I experienced anywhere such shoddy reliability and utter incompetence in running a service as I have in Bristol, and it's been like that for years. I can't really comment on the rest of the First West of England operation, but based on the limited experience I've had, at least I've actually gotten to my destination!
Diamond Bus South East are also a truly atrocious operation, I've found. Aside from the fact that they don't bother to respond to any of their emails, the kind of service they provide on the buses themselves makes me think that being surly, rude and having zero customer service skills must be listed as a desirable quality on their driver application form.
Not to tar all of their drivers with the same brush, however, as I have met a handful of lovely drivers, but the majority I've encountered with this company exude zero professionalism and seem to take joy in belittling and arguing with you about tickets which they are clueless about. Evidently ticketing training is one aspect the company has failed at, not to mention customer service training.
In terms of reliability, Stagecoach South (particularly in the Guildford area) are a nightmare to travel with at the moment, with services constantly being cancelled at short notice day after day due to driver shortages - often with consecutive "hourly" services cancelled, leaving you to wait two, three hours just to get home. When they do run, however, they are rather pleasant to travel with.
Definitely agree about Arriva Southern Counties - a right scruffy mess. During their final few weeks before they pulled out of Guildford, you'd be forgiven for mistaking them for the ropey cowboy independent - disgustingly filthy buses held together by gaffer tape, appalling reliability, drivers wearing hoodies...
(Pictures attached show the state of one of Arriva Guildford's buses on one of my final journeys with them!)
On the flip side, White Bus Services - also in the same general area - are an excellent small operator, with fantastic reliability, friendly drivers and spotless buses.
Similarly, Falcon Buses have improved (and expanded) massively in the past couple of years and are also a very smart little operator.