I'd say stagecoach, though I may be biased as I work for them. Go NE certainly have the nicest fleet and are the cheapest but, certainly around Sunderland, Stagecoach seems to be much more stable and better in actually providing a service - they aren't perfect, but still fairly good in my experience, and the buses are so frequent (exceptions being the X24 and 8) that you're not delayed too much if a bus is cancelled anyway. Also they pay their drivers the most. Can't comment on my experiences of Stagecoach elsewhere in the NE because they're non-existent.

I used Arriva during college and learnt very quickly to avoid them like the plague. I have no reason to believe that's improved considering they seem to be chucking whatever they found round the back of the shed on the "Sapphire" 22/23/24. I've seen bus types on those routes that I never even knew existed.