I have every album including the bootleg rarities like Hormoaning, Outcesticide I, II, & III, and numerous live albums. In Utero is definitely my favorite album. The drum acoustics are probably the best I've ever heard on a record.
These days though, when I get the craving for some Nirvana tunes, I usually listen to the rarity albums instead of the mainstream ones. Songs like "In His Hands," "Sappy," "Return of the Rat," and "Gallons of Rubbing Alcohol Flow Through the Strip" are all awesome, as is the title song on the greatest hits album, "You Know You're Right."
The pinnacle of their career quality-wise was definitely '94 when they recorded the MTV Unplugged session. That and In Utero showcased just how much better the band had gotten after the three-chord pop songs of Nevermind. I just wish we could have gotten one more album out of Cobain before the end.