If you have money to spare, and can get your hands on a few more Japanese re-releases (I think the entire catalog of YMO has been released on a Canadian label, but then these are most probably out-of-print), then the next ones to go for are Naughty Boys Instrumental and BGM. Avoid Multiplies unless you have a fetish for sick humor, and avoid Service unless you want to listen to long radio-dramas in Japanese.
Their reunion work Technodon (and the live recording that followed) is wonderful. Playful, humorous and lush with beautiful sounds.
They have loads of live albums (some may be bootlegs) that I'm not altogether familar with.
I always think, compared to Sakamoto, the music of Haruomi Hosono is truer to the spirit of YMO -- but then many of his CDs are Japanese-only, some may be out-of-print. I recommend The Story of Genki, Naga; and if you want something with a beat, then N.D.E and OmniSightseeing
Sakamoto is very diverse, and if you are looking for overtly electronic, YMO-like stuff, you'd better go right back, just after YMO disbanded, for albums such as Musical Encyclopedia (get only the Japanese 13-track version; the 8-track international version misses out most good stuff), and perhaps Esparanto (sort of "musique concrete", not for everyone). Although Sakamoto do put out electronic albums all along (e.g. Beauty, Heartbeat and Sweet Revenge), these are more popish, more radio-friendly music, and at least in the case of Heartbeat, bowing down to the musical fashion of the time.
