Hmmm...
Apocalypse War
Build up: Block Mania and Pirates of the Black Atlantic
Aftermath: Anything with rad-pits, rehoming citizens and that one with the robot wrestler. Plus anything related to the Sovs/Orlok.
Necropolis
Build up: There's an argument for all the democracy stuff and bloodlines stuff, plus The Deadman, Tales of the Deadman, Countdown to Necropolis and, arguably, all the Dark Judge stories up to that point.
Aftermath: The referendum, Dredd without a nose, Silver's return, Judgement on Gotham, the burial pits in Judgement Day, Young Death (and thus Mrs Gunderson), plus a good few one offs (including a great PJ Maybe).
Simply for the thrill of reading it at the time, and it being the end of 'classic' period Dredd, I have to go with Necropolis.