If I could only pick one it would have to be Google Analytics.

What Jill said (though I'd hope with my Google account, I'd also have access to their AdWords & Webmaster Tools).

It would have to be Raven for SEO. Just because of it's swiss army style offering. There are far more indivdual powerful tools, just as there are much better seperate knives instead of the swiss army knife kit, however on a desert island you'd just want one holistic solution.

It's truly hard to pick anything better then Google Analytics.  Maybe microsoft outlook so that when I create fantastic content I have the opportunity to share it with others and hopefully generate links.

That's so tough, I'm going to go with a browser-based one, Quirk SearchStatus. PageRank info, nofollow highlighting and much more.

I would go with the SpyFu Suite (if I can get away with a set of tools in one) otherwise just SpyFu Kombat. Competitive data is helpful when they (the competitors) have the tools that I would not have and be able to gauge their success through the other data available there.

Sorry Todd, you don't get Webmaster Tools and the keyword tool, too. :-)

I'd also have to pick Google Analytics. Or some analytics tool. I'd like to think that I could use my brain and hard work to make up for the lack of other tools. But I don't know how I'd make up for not knowing what is actually happening across my site.

This may not be the most intellectual opinion expressed here today, but if I'm stranded on a desert island, my clients can suck it.

Google AdWords Editor.

DrPete does have a good point...

The SEOmoz suite, including the Labs tools.

excel

Access to my Odesk account. :-)

webmaster tools i think

opensiteexplorer.org would probably be my one thing, but its a tough call