For me, the most disappointing car of the decade is the SLR McLaren. Offering a ompromised handling and driving pleasure (for such a car, of course, i don't mean it's less fun than a normal car) are pretty much the worst that can happen to a supercar.
As close second comes the Maybach. It's such a departure from the legendary Grosser Mercedes (770, then W100 600), both of which representing the absolute best mankind was able to offer in terms of vehicle at the time... While the Maybach is few more than an inflated old-gen S-Class with a rather uninspiring interior design...
The Prius is an outstanding car. It's imo one of the cars that will mark the decade. It's no E-Class, never intended to be. It's a rather small, not very handsome, but highly technological car and imo still represents the best an hybrid can offer.