The worst offenders are the Quest, Maxima, and revised Sentra. The Armada isn't great either.
The '05 Altima looks very good (even on the inside!), and I actually like the Murano. The Xterra's plasticky up front but overall a good-looking vehicle IMO. I think the 350Z is distinctive in a good way, and the other new trucks (Titan & upcoming Frontier and Pathfinder) look good too.
There are few mainstream car companies that don't have at least a few ugly cars in their lineup, though. The big 3, Toyota, Honda, Mitsubishi, etc. all have a few design lemons or more in their lineup. Mazda probably comes closest to having an entire lineup of vehicles that look good.