I feel prompted to post, because I don't recognise the above comments describing the casefiles (in general) as being true at all..only containing one off stories? eh? No offence, but it sounds like someone has read the first and given up. I've been working through them (up to 12 now from the beginning), so I can safely say that the posters opinion is only true of the first volume, which covers the early days when Dredd was finding its feet.
Since the casefiles document all Dredd stories, they are the true Dredd representation and as they progress you are getting all the huge, epic and as time progresses quite adult stories in there.
In my opinion Dredd Year One is really not the best representation, and you are best off getting the real deal - it's an effort by a interloping writer who doesn't really get what Dredd is. It's not even by Wagner/Grant. I also felt Batman/Dredd stories are essentially played for laughs and do not really feel like either Batman or Dredd stories, certainly not the latter. Just my opinions though, your mileage may vary.
I would pick up Casefiles 4 for the Judge Child Quest, certainly 5 for the incredible Block Mania and Apocalypse War arc. 2 is also of interest for The Cursed Earth saga, but its not as incredible as people say. After that, up to you, since the casefiles are consistently decent from that point.
But look for the big sagas such as Necropolis, Day of Chaos etc.
This page is a great place to start:
http://io9.com/5944097/top-11-essential ... dd-stories