A cache that drove me bananas... Hmm... Based on difficulty, there are quite a few. Based on importance to myself and how it became all-consuming was the NC DeLorme Challenge and Tube Torcher II.
Currently, there is a difficulty 1.5 cache in my area that I just can't find, "Game On". No matter what I do, I just can't locate it. It's a curse.
The TB owner is "2 Blind Squirrels". I'd love to give this TB to the "Blind Acorn" in Roanoke, VA. That would be funny.