p.2 #1 · What Canon lens holds its resale value the best?
I bought a few lenses from the US and sold them some years later in Europe. Those lenses were sold for at least their original price, if not more. 70-200mm f4 IS was sold for roughly the same amount, while the 500mm f4 IS was a good investment, but I bought it used.
Now, I don't think that's still possible. So, I guess it depends on some variables.
p.2 #3 · What Canon lens holds its resale value the best?
The 500 f/4 II is doing pretty well, I think. It's only been about two and a half years since I bought mine used (~$6500), but they're still going for about that same price.