Well, I've only been in California for a few months now, but I've never been to Nevada, unless you count an airport layover I once had in Las Vegas. If that counts, then I guess the answer would be Oregon.
It's also an interesting philosophical question if you have been to a state if you've flown over it.
When I lived in Australia, I made two trips to Europe, with flights: Sydney->Bangkok->London on one trip, and Perth->Kuala Lumpur->London on another. I thereby flew over much of Eurasia, so if flying over is good enough, then I've "been to" India, Pakistan, Afghanistan, Kazakhstan, Russia, etc. That seems like cheating though.
