Since this desert island obviously has electricity, I'm talking all of my real drums and amps and will just record there like I do here. No magic VSTs required.
The one plug that's on everything these days is the Eventide Ultraverb. Every time I stick another brand on I eventually switch it back. I have been using a lot of Nebula recently which opens up a whole lot of territory.
Cant think of one I use more than others or couldnt do without, I love the presonus fat channel, I generally have it on every channel whether I use it or not...the new Uad fairchilds are excellent too.
I don't think that I could pick just one. I have several that I use on every project. But the one that seems to consistently amaze me is Waves L1 Limiter + Ultramaximizer. One-knob hard limiting is perfect for home-brew "mastering". It's so transparent and makes it simple to bring mixes up to volume for sharing. I would give up the rest of the Gold bundle if I could just keep L1.