I would recommend not buying a 5 pound jar of mayo, since it will go bad long before you get through half of it. Per pound, it may end up more expensive because of the waste.
This article has a solid list.
Our family consumes quite a bit of peanut butter, and that is one that we buy in bulk, especially when it has a discount at Costco.
Does anyone finish 20 jars of pasta sauce? Buying in bulk is a smart way to save—if you buy the right items.