I would probably go with either knife or first aid kit. The first aid kit would be to keep myself from getting infected and dying on the island, but I'd probably go with the knife anyway, because most same as was said above.
Of course, it would be a nice big knife, like a machete, not just a little swiss army knife.
As for people, I'd hate to take anyone with me for their sake. But for my sake, I'd probably take a friend from high school who has been in the military and in Iraq for some time now. He'd probably know a few things about staying alive.
Of course, my alternative would be my brother in med school, since he'd (hopefully) know what to do if one of us got seriously ill from something on the island.
I've learned a little bit about wilderness survival myself, but I've never had to put it to the test, and hopefully (and most likely) never will.