My vote goes to the Dingman's Ferry Bridge which spans the Delaware River between Milford, PA and Sussex County, NJ. You can actually see all of the paint marks along the side of the bridge from cars who have scraped the guard rail. Narrow is an understatement.
Not really that scary. I've crossed the bridge in the area of 120 times over 19 years. The Dingman's Choice Bridge Company does a good job maintaining it and each Labor Day closing it for maintenance. Remember the the bridge is almost 110 years old. (110 will be in November)
Pond Eddy Bridge to most people is scary, but I've gotten used to crossing it without problems, I mean its a 107-year old relic. It needs replacement, but who knows what's going to happen. No construction is supposed to begin until
2013, when the bridge has its 110th anniversary as well. Its long outlasted its predecessors.
Some of the bridges on the northern end of the Delaware (when its a navigable span), both the Skinners Falls - Milanville Bridge and the Little Equinunk Bridges are one-lane, very tight, large spans. Although they may not look so, both are very well maintained and very old each. Skinners Falls is 109, while the Little E is 121, which is a nice tribute to all who honor the Delaware to have such old bridges.