I would not recommend motorcycles for stop & go traffic. They're too easy to overheat, especially if you get an air cooled one. Hybrids are made specifically to perform well in bumper to bumper traffic. Otherwise, your best mileage would be in something small like a Geo Metro or Honda Civic VX.
I think a water cooled bike with an auxiliary fan won't overheat in any amount of traffic jam. Other than that, I'd recommend small and light ones, like a 125cc. Slow speeds and miserable acceleration of the queue won't need anything strong, and small ones are definitely more efficient at low speeds and idling. Low weight comes handy when you have to brake: you won't throw too much momentum away.
Bikes' trump card is that you can split lanes on multi-lane roads, so you can be much faster than a car that just sits there idling or not. If it's legal in your area, I mean.
OTOH, hybrids are really made for such situations, and they won't unnecessarily idle when waiting.
New, the Fusion has an auto start/stop option. There is also a Kia and maybe a Hyundai available with it. Installing a second battery would get more out of it in that much traffic. I have seen reports for owners overseas of Kia's with it have to get the battery replaced sooner. With heavy use of any system it would be prudent to regularly charge the battery.
Mazda doesn't seem to be offering their auto start/stop system yet, but the new 6 has their i-ELOOP regen braking system as an option. It increases the city rating by 2mpg. It also has active grill shutters for 40mpg rating on the highway.
I'd like to exclude those [hybrid and motorcycle] vehicles in this thread though.
I'd concur the Civic VX would be a great choice, although shifting out of 1st and 2nd gears might get tedious. If it gets really slow, I sometimes will turn my car off and just push a little and if you are on a gradual decline that is barely moving you can also turn your car off until you get to the bottom of the slope.
I have a few fuel saving tips when going through traffic jams.
-When the speed is picking up ahead: move closer behind the vehicle in front of you.
-When speed is different between lanes or traffic is slowing down ahead in your lane: keep a greater distance. Brake some early if you have to. The optimal is to arrive close up to the vehicle in front of you just as the speed starts to pick up in that lane, to avoid people changing lanes in front of you and keeping your minimum speed at a maximum.
-Never drive behind a large truck that blocks the view ahead.
Other than that: smallest engine and lightest car possible will increase FE in traffic jams. Preferably a diesel which has lower consumption when idling.