I'm not going to argue with Batman #1's first place slot, thanks in large part to the significance of the characters it introduced, but in terms of enjoyability, a very good underrated book (cover-to-cover) in my mind is the 1939 NY World's Fair.
I have heard others speak fondly of it but I've never held one. What makes it special to you?
Every story is actually written with the Fair in mind. Clark & Lois are covering the Fair as reporters when Lois gets kidnapped, Slam Bradley breaks up a gang of crooks on the NY fairgrounds, Zatara's at the Fair, hell, even Bob Kane's Ginger Snap has a humorous adventure at the Fair. The book has a really nice flow to it, it seems like a great slice of life from that time period, plus you get the first ever Sandman story by Gardner Fox and poor Bert Christman.
It's a great read, and a reasonably significant book. Yeah, it's cold as ice in the marketplace, but I'm a fan.
Win, have those stories ever been reprinted?
Sounds like a great read and one of the reasons I enjoy the Fantom of the Fair stories so much.