If by imagine you mean visualize, then any geometrically impossible object would be hardest to imagine.
Unless your last name is Escher. ;D
But since I can imagine sounds and other senses...in the truest sense, your question cannot be answered by me in any language I know. Any answer true for me would have to be in a language I do not understand, embodying concepts English has no equivalent for. By definition, anything possible to describe using one's natural language can be imagined, even something as abstract as "an emotion" or "politics," because one imagines the physical events/objects surrounding the concept as a representation of it.
So, in a broad sense, your question can never be truthfully answered in the same language that it is written in.