Well, I think amarillo is one that doesn't really have any good subs. Nor does simcoe. I also think chinook would be hard to replace. Northern brewer is another one that I don't think I would be able to sub for, with the same results. As I think about it, I think they all are unique and have slightly different qualities but there are more similarities between hops from a certain region.
For example, German hops have a similar profile; halletauer, and tettnanger have similar flavor. English hops like fuggles and East Kent goldings are similar in their characteristics, and US hops like cascade and centennial are both citrusy. I can't think of any that are a great sub for each other, but there are some that can be close enough that it may not be terribly noticeable in the finished product.