Since i've started reading GNs/CBs way back 2015, and collected around 150+ books (both in HC and TPB), and IMO basing from what i've read in 3 years of GN/CB collecting here's my top 20 GNs/CBs of all time (in no particular order)

Batman: The Killing Joke by Alan Moore & Brian Bolland

V for Vendetta By Alan Moore

Watchmen By Alan Moore & Dave Gibbons

Preacher By Garth Ennis & Steve Dillon

Animal Man By Grant Morrison

Doom Patrol By Grant Morrison

Invisibles By Grant Morrison

Flex Mentallo By Grant Morrison

Batman: Year One By Frank Miller

The Dark Knight Returns By Frank Miller

The Sandman By Neil Gaiman

Transmetropolitan By Warren Ellis

Planetary By Warren Ellis & John Cassaday

100 Bullets By Brian Azzarello & Eduardo Risso

Y: The Last Man By Brian K. Vaughan

Ex Machina By Brian K. Vaughan

Fables By Bill Willingham

Hellblazer

DMZ By Brian Wood & Riccardo Burchielli

Scalped By Jason Aaron & R. M. Guéra