Most Motown artists - the Supremes Gold, for example. Or just get the Hitsville USA box set.
The Pet Shop Boys Discography.
Billy Idol Vital Idol (he's hardly "essential" to anyone, but this is the only album you will ever need by him)
Elton John's Greatest Hits (1975 edition - he quickly turned to crap after that, so ditto above)
Madonna The Immaculate Collection (ditto ditto)