Varies way too much mate, what the best black box company for me is, may be the worst for you.
Too many factors - experience, location, crime levels, etc etc.
Best advice you could possibly get is use comparison websites if you aren't already.
Once you fill in the car and your personal details, you'll generate quotes and companies will pop up with whatever curfews and stuff they may have.
Majority of them don't really have curfews anymore though, mostly speed and driving controls.
I was with AXA for example, no curfew and allowed me to go 50% over the speed limit.
Don't go 50% over the speed limit..