That's an extremely broad question. Quarters have been issued since the 1790s so there are many valuable issues, and the "most valuable" title can shift as time passes. Some of the highest-value quarters are 1796, 1870-CC, and 1871-CC. You can find a full set of current prices at any number of web sites. One good portal site is http://www.coinclub.com/prices/
By far, the most valuable quarter is the 1916 Standing Liberty Quarter Dollar - its value in good condition (G4) is: $3000. places the value of the 1916 Quarter at $3900 in G4 and $35,000 in MS66 (but look fast, it may be higher by the time I finish typing this). The first US quarter from 1796 is listed at $12,000 and $400,000 in those grades.
From least to greatest: Penny Nickel Dime Quarter Half Dollar Gold and Silver Dollar
The most valuable American coin in current production is the half dollar. This half dollar is worth 50 cents when used.
1895
Quarter a dollar (1/4 a dollar)
Depending on the grade, values are $3.00-$8.00 for most quarters of this date but can have values as high as $50.00 or more.
I believe they are all worth 25 cents...
The most two most valuable are the 1903-O Morgan and 1928 Peace dollar, followed by the '21 Peace and the '02-S and '03-S Morgans.
The USA has a 25 cent coin which most people refer to as a "Quarter", ie. one quarter of a Dollar.
Most accounting systems list all transactions for the quarter in which they are actually paid, regardless of payment due date.
1970-S
The term "Silver Dollar" covers 7 different series (the coins that actually contain silver) of one dollar coins struck by the US from 1794 to 1935. Please be more specific and post new question.