WEll there are plenty of one off items (the helping hands spring instantly to mind.Also the drum head John sold with all the bands sigs on it) and as there is only one of any particular one off (makes them one offs I suppose
) it's impossible to get more rare than that.
But to answer the question of what is most expensive, you have to look at a factor which is even more important than how rare something is.
Desirability.
For instance, does anyone remember the raindeer Meat painted for charity?
Quite charming in it's way, but I don't think too many people would go a whole bundle on the item now it's long ago and far away.
The hype that originaly surrounded it has gone.
Of course desirability is down to personal taste, which means that the ulimate collectors item (and i'm guessing that's what this thread is out to find) must be something that is very rare yet something that most fans find very desirable.