Last edited by K7P5V; 05-22-2019 at 07:44 PM. Reason: Made a correction.
Your points about Watchmen are spot on. The series is brilliantly technical and crosses all of its Ts and dots all of its Is. One could argue the actual story being at the top of the heap, but the detail with which it was created is something I haven't seen duplicated. I welcome mention of any other stories that anyone thinks matches or surpasses Watchmen's technical detail.
I think for that you'd have to leave the world of superheroes and look at "serious" comics. I'm not sure if these match Watchmen in terms of clinical precision, but here are a few books with more going on than meets the eye:
The Airtight Garage by Moebius. Also The Gardens of Aedena, particularly the later books. Moebius was the opposite of Moore in various ways. Just as Moore was the master of conscious precision, Moebius was unparalleled at intuitive, subconscious creation so they contrast nicely.
The Dormant Beast series of GNs by Enki Bilal
Cerberus by Dave Sim (at least up until Melmoth, after which it goes thoroughly off the rails)
From Hell by Alan Moore and Eddie Campbell. Maybe the only real competition with Moore is Moore himself. I think this is actually Moore's best work and it shares some themes with Watchmen.
Books of Magic
Black Orchid
DMZ
Green Lantern Emerald Dawn
America vs Justice Society
Hawkworld
Time Masters
Omega Men