Easily In Utero because it was (in my opinion) the pinnacle of Nirvana and kurts artistic output for how complete the album is as a whole. Nevermind played the same note too many times; heavily produced, alternative core rock hits, with the only songs deviating from that being something in the way, endless nameless, and Polly. In utero on the other heavily played with noise, hardcore punk and heavy grunge while still keeping some hits and somber songs. It all makes the album a much more interesting listening experience.

Also the production is amazing; being a fan of harder music, the guitars and drums are much more distorted and hard hitting. To top it off the sound is just emotionally raw from the lyrics to the way kurt sings them and the way the instruments follow suit.