Pretty basic list, now that I write it down--but here's my top 12, with 2 honorable mentions:
-Prisma Sonoro - Alessandro Alessandroni
-"Faralla" - Alessndro Alessandroni
-Open Air Parade - Alessandro Alessandroni & Giovanni Tommaso
-Nel Mondo Del Lavoro - Rino De Filippi
-Tempo Sospeso - Bruno Nicolai
-Voix - Egisto Macchi
-Citta Notte - Egisto Macchi
-Genti E Paesi Del Mondo - Piero Umiliani
-Pop Impressions - Janko Nilovic
-Rythmes Contemporains - Janko Nilovic
-Mindbender - Stringtronics
-Voices In Harmony - John Cameron & Keith Mansfield
-Le Coeur En Fete - Andre Popp
(technically not a library--but a gateway record for me into similarly sounding music, and I love it dearly.)
-Inner Space - Sven Libaek
(soundtrack, but another Ur-record of the library sound for me. All those mallets and Libaek's jazzy sound drive me crazy.)
Besides heavily favoring Italians, I guess my taste breaks into two (maybe three) sound camps. One is the lush orchestral pop that Alessandroni nails--and that others like Nilovic get at from a brass-ier direction. Especially, I go weak whenever a harp, bell, or other plucky sound gets perfectly integrated with strings or wordless vocals. The second camp is the more abstract classical style of Macchi and others. I like more fleshed out compositions with a sense of melody, ambience, mystery.
I'm a bit prejudiced against electronic instruments in library albums--anything to where I really start to notice. I wonder if maybe I'll come around eventually since, objectively, I enjoy a lot of it. But...the color just isn't there, and it isn't the sound of "Library" to me. Does anyone feel similarly? (or the opposite?).