Which album titles do you feel no good CD collection should be without? I'll start with:

Tapestry - Carol King.

depends the type of music.

For the types of music I listen to, here are ones for starters I think fans of the genre should own

rock (classic): (I go for studio albums instead of hits collections since Classic Rock is very ALBUM oriented)
AC/DC - Back In Black
Beatles - Sgt Pepper
Led Zeppelin - IV (then get II)
Pink Floyd - Dark Side Of The Moon (then The Wall)
David Bowie - Ziggy Stardust

rock (last 15 years or so):
Nirvana - Nevermind
Weezer - Weezer (the one from 94)
Guns N Roses - Appetite For Destruction
Coldplay - A Rush Of Blood To The Head
Pearl Jam - Ten
Alice In Chains - Dirt

pop (classic pop, not todays stuff):
Madonna - Like A Prayer
ABBA - Gold
Elton John - Greatest Hits
Paul McCartney - All The Best (or Wingspan, but Wingspan leaves off a few 80's smashes that are on ATB)
Duran Duran - Greatest
Elvis Presley - The #1's

classic r&b
Marvin Gaye - What's Going On
Michael Jackson - Thriller (then move on to Off The Wall)
Stevie Wonder - Original Musiquarium
Donna Summer - Endless Summer
Supremes - Ultimate Collection
Prince - 1999 (then move onto Purple Rain and Sign O The Times)

dance music:
Donna Summer - Bad Girls (Deluxe Edition with the 12" mixes)
Madonna - Madonna
Janet Jackson - Control (and if you like that, then get the Control The Remixes, which is a nice remix companion piece)
Pet Shop Boys - Very
Kylie Minogue - Fever
Saturday Night Fever soundtrack
Dead Or Alive - Evolution

those who like women who rock (put on any of these and you'll be shocked at how POP Avril really sounds):
Joan Jett - Fit To Be Tied
Pat Benatar - Crimes Of Passion
Heart - The Essential Heart
Liz Phair - Exile In Guyville
Linda Ronstadt - The Very Best Of
Babes In Toyland - Fontanelle
Blondie - The Platinum Collection
Pretenders - Pretenders

new wave:
Elvis Costello - My Aim Is True
Talking Heads - Remain In Light
Human League - The Very Best Of
The Cars - Just What I Needed:The Cars Anthology

singer-songwriters:
Billy Joel - The Stranger
Carole King - Tapestry
Tracy Chapman - Tracy Chapman
Ryan Adams - Gold
Joni Mitchell - Court And Spark
Kim Carnes - Gypsy Honeymoon

divas:
Barbra Streisand - The Essential Barbra Streisand (or The Barbra Streisand Album if you wish to get a studio album instead)
Diana Ross - Diana
Bette Midler - Experience The Diving
Cher - the imported version of The Very Best Of Cher
Aretha Franklin - Aretha's Best
Mariah Carey - Mariah Carey
Tina Turner - Simply The Best
Olivia Newton-John - Magic:The Very Best Of ONJ
Dolly Parton - The Essential Dolly Parton I & II
Donna Summer - Endless Summer