with the Joker, it really depends on which backstory you use for him. If you go by The Killing Joke's backstory, I'd say Joker is; he may be a far worse villain than Osborn (I have no idea), but that particular backstory can at least make you pity him a little bit for becoming what he is. Osborn, as far as I'm aware, was a normal evil rich guy who made himself into a superpowered crazy evil rich guy. Not much of a sympathetic angle there.
Then again, I just read on wiki that he was abused as a child for the express purpose of making him a ruthless businessman, so I guess that about evens things out. I don't think either one of them is commonly represented in such a way as to draw attention to the sympathetic aspects of their respective characters though, so really neither one is particularly sympathetic. I guess I'd have to give the edge to the Joker there simply because he has The Killing Joke as such a stand out story.