I don't think you can pick one for Crue because they have different albums defining different aspects of the band. Feelgood is the biggest seller and probably the one the casual pop & rock fan remembers the most, however most people who listen to Motley probably remember them more fondly for Too Fast For Love or Shout At The Devil. No doubt Feelgood is the biggest but I wouldn't say it represents the band like the first 2 do, seeing as though Motley are more remembered as being the crazy big black haired stiletto heeled drug addicted tattooed bad boys of rock & roll and pioneers of Sunset Strip glam, rather than the clean sober fun time rock & roll band they were on Feelgood.
Kinda same case as Metallica, Black & Puppets are both defining albums for different aspects of the band... though Black is the biggest seller it's not their one defining album.