I've not read the comics or Vol III so I can't comment on them but, although Vol I has the best collection of stories overall, I think Vol IV had some of the best stories of the whole series.
I think my favourite comes from Vol II; The War Between the States. NYC, flappers and a love triangle = awesome. Loved that it wasn't written from a slayer/watcher's/demons perspective too. Great insight and really sad.
I also really enjoyed Alone (Vol IV - Catherine, an Irish Slayer), Mornglom Dreaming (Vol I - Mollie, a Kentucky Slayer) and It's All About The Mission (Vol IV -Nikki, a NYC Slayer).
The only thing I find a little annoying is that the majority of slayers all tend to be from America or Western Europe and existed in the last few centuries. I'd love to read about a Slayer from India, Russia or South America who existed in the dark ages or something. I suppose it must be hard to do justice to a story when the culture and times are relatively unknown - and the more distant and exotic, the tougher the research - but it would be fascinating.
I'd love to know about the rise of the Council too; how did it develop from the original three magic men to the patriarchal English institution (I bet the Empire had something to do with it.....)?
Some of the Slayers must have resisted the Council's control too (for patriotic reasons apart from anything else), or fought their calling, or got a bit drunk on the power (perhaps not as madly as Faith), or went mad with the pressure, or the Council thought a Slayer so unsuitable they 'eliminated' her (eeeek) etc.....I'm desperate for that sort of stuff to be explored! 🙂