Agree with you 100% - Abbey Road is The Beatles' best and most underrated album, although The White Album and Revolver are not far behind. Always felt St. Pepper was a little overrated but still a great album.

Yeah I doubt after all this time it will be outright bad.

There are plenty, plenty more. It would be more difficult if it was "never made an average album" but no terrible albums from the following:

Björk (as Björk)
PJ Harvey
Nick Cave & The Bad Seeds
Kate Bush
Radiohead (although Pablo Honey has dated badly)
Portishead
Nine Inch Nails
Opeth
Mastodon
Dream Theater (haven't heard their debut)
King Crimson
Pink Floyd (The Endless River maybe the exception)
Talking Heads (probably, haven't heard last two)
The Cure (some average)
Depeche Mode (again, some average)
David Bowie (debut and Pin Ups are not supposed to be great)
Loads more...