Well first we have to eliminate the things that actually have a use. Anything that does damage or has a strategic value, however small, can be considered to have a use and therefore the sz15 DD, Gibraltar, US bomber, SFE, W Can inf, and German WR units must be removed from contention.
Midway and Wake are eliminated because they both prevent an infantry from drowning and therefore carry the value of an infantry whether they are used or not.
In the past I have used sz26 so it is out.
Which leaves us with Greenland and Crazy Ivan.
I have never used Greenland. Not even for a coaster. But, the question asks which unit/territory on the board is most useless. Now, in certain circumstances, Crazy Ivan can be used to represent a unit as a plastic chip but I would feel compelled to give him an atomic wedgie for standing on my board. HOWEVER, Crazy Ivan does have the capacity to bribe me not to vote for him. So, I will put Crazy Ivan to the side for now pending receipt of payment.
In conclusion, the answer is most likely Greenland. The ball is in your court Crazy Ivan.