Here are some demos I consider classics, regardless of whether they are perfect or not. This archive is full of demos from the early Amiga demo era, and runs best on an Amiga 500 with 512k extra memory, or at least an OCS/ECS Amiga with some fastmem.
You can run them directly in an emulator, of course. This is also useful to find out which disk images you want to write to floppy on your Amiga or Gotek drive!
500 disks, thousands of demos. Get the Demo Disk Archive from 'Demos' on the Downloads page! (Update: an ADF-only version has has been added. Get that if your device can't handle DMS.)
Some knowledge is useful to get all of them running correct. Read the guide!