I would not place "The killing joke" as a "best" graphic novel - it wouldn't be in my Top Ten - I am not even sure it would make the grade for (my personal) Top 100.

What I ALWAYS recommend as a timeless classic, and as it is written by Alan Moore - so you should be interested, is the "Saga of the Swamp Thing" Volume #1 to #6.
I read it in floppies when it was published and over the years often dug out those copies to reread them.
The recently published HCs (I understand, that they are also available in SC) are a good way to get them for your personal library.
I read them again in one fell swoop and boy, they are still exceptional great. Wonderful scripts and the art is topnotch - a real shame Bissette isn't drawing anymore!

Go get them - NOW!