Well there have been two $ coins that I am aware of in recent times. The Susan B. Anthony, which as mentioned looks more or less exactly like a quarter, and so you get rid of them as soon as you can (while you still remember you have one/some). They are so hated, that you feel a bit bad inflicting them on a shopkeeper.
http://www.usmint.gov/faqs/index.cfm?action=FA QSearchResult
The other is the Sacagawea introduced in 2000
http://www.usmint.gov/mint_programs/golden_dol lar_coin/index.cfm?action=sacDesign
I like this coin, you know if you have a few of them in your pocket, they are chunky. When they were introduced I remember one of the mega chainstores (Walmart, Costco? something like that) was giving out all singles in Sacagawea's. I think it was part of a campaign to get them accepted by the public.
For some reason Sacagawea's just do not seem to have taken off. If you need to put $5 into the parking meter at your local train station, you're still going to need to rustle up 20 quarters. Most vending machines were never converted to take dollar coins.
Both the Susan B Anthony and Sacagawea are in circulation, but the only time I get them is in change from train ticket machines.
My conclusion is that the Americans are VERY conservative about the form of their currency. But to answer your question, although you still see them occasionally, I suspect the answer is the Susan B Anthony.
p.s. Now if you had have asked about US$ bills, I've have said the $2 bill... I still remember those from the 70's...