My own recommendations.
For Dredd I’d go for Case Files 02, 05 and 14 for the Cersed Earth, Apocalypse War and Necropolis stories respectively (if you are going to read Necropolis it would be a good idea to pick up and read a collection called The Dead Man first too). Those are three of the best classic “Mega Epics” in my opinion. Out with these Case Files I’d pick up Dredd vs Aliens: Incubus and Dredd: Origins (titles really describe all you need to know). The Dredd / Batman collection is also a lot of fun too.
For a slightly more human view of Dredd’s world I’d recommend Judge Anderson: Shambala. Stories of life, love, religion and death in Mega City One all coupled with Arthur Ranson’s unparalleled art work.
a couple other recommendations outwith Dredd…
Defo: post English Civil War steampunk-ish zombie hunters. Lovely black and white art work and some real depth of research put into that period of history in the writing.
Zombo: about a government created zombie in a red thong. Hard to really describe beyond that other than saying it’s completely demented.
Shakara: Interplanetery vengeance tale. Artwork is stunning in black and white and occasional colour.
Loads of other stuff but I’ll leave it at that for now (and give others a chance to chime in too!).