Difficult question... If I could include EP's, i'd place Random 1-8 above all of the studio albums, But I tend to go more by "era" than album (including b-sides), so that's what i'll do here.
Showbiz (Wouldn't necessarily say it's their best album, but the B-Sides are simply amazing.)
BHAR (Just a bit too weak for my liking. Songs like SBH, Starlight, Motp and KoC just don't do anything for me. Although I do love the songs Easily and Invincible)
Origin of Symmetry (just my opinion, but OOS is very overrated.)