Leaf Hops. Especially when you buy several varieties cheap by the lb. and they take up all that freezer space. Think about how much you use hops in American style hoppy beers: FWH, Bitter, Middle Additions, Hopburst, Whirlpool, Hopback, and/or Dryhop.
I'm thinking a lot of people hoard 2-row and/or DME. However, I don't buy them in bulk. I plan out my recipes and buy just what I need when it comes to fermentables. I don't have a grain crusher yet so it's just more convenient for me to buy what I need instead of letting the crushed grain go stale.
Not so much yeast, unless you're washing it, capturing, or breeding it, and saving it in mason jars.