If you're talking about the older issues called sovereigns, there are many valuable dates. See the Related Link for values (note - all prices are in £ sterling).
If you're referring to modern £1 coins, none of the standard issues are particularly valuable. Proof examples may be worth $15 to $30 in some cases, and of course special silver and piedfort proofs are worth more - $50 to $200.
The rare dates are as follows:
1847
1848
1852
1858 with BRITANNIAB error
1858 with the last 8 struck over a 6
1869
1882 (rarer than originally thought as the Royal Mint was closed for most of that year due to a change of machinery)
1893 with Jubilee Head
1942, 1943 and 1944 are scarce in mint state
1945 - most were melted down but it appears a handful exist - excessively rare
The are no nickel-brass dodecagonal (12 sided) British Threepences considered to be rare.
A date having a significantly higher collector value than all others would be the 1946 coin, especially in mint uncirculated condition.
No British brass Threepence is considered to be rare.
The years when the fewest were minted are -
1949 - 464,000 minted
1946 - 620,734 minted
There are no rare Sixpences from 1928.
Sixpences minted between 1920 and 1946 were struck in 50% silver. They therefore have a scrap value by weight of approximately £14 per Troy Ounce (as at October 2010). In mint state they have a collectors' value or numismatic value. Dealers often offer a multiplier of the face value for them i.e. at the moment 10 times their face value would be an approximation. Sixpences struck before 1920 i.e. 1919 and earlier were struck in 92.5% silver and will fetch almost double. There are rare dates of course, 1893 with the Victoria jubilee head recently fetched £5,250 in mint state due to its rarity.
If you refer to British general circulation Threepences, there are no rare coins.
The only British Sixpence that could be considered valuable would be the 1952 coin. A British 1952 Sixpence, uncirculated and in absolute mint condition could fetch up to £95 GBP. If it has been circulated but still in good condition, it might fetch anything from £5 to £30 GBP. A reputable coin dealer will be able to give a more accurate valuation.
With no rare dates they likely are only valued for the silver.
There were 5.578 million 1933 British 50% silver Threepences minted. They are not especially valuable or rare. If you have doubts about the authenticity of your coin, take it to a reputable coin dealer for a positive identification.
It is not rare, just expensive. But, you can get small silver coins for as little as $2.
A rare Morgan Silver Dollar is 1876.
It might be, I have never heard of one. One tenth of what? What year? Who and/or what is on the coin? What diameter is the coin?
A 1941 silver threepence is only 50% silver (assuming for a moment we are talking about British coins) and as such is really only worth a bit less than a dollar in scrap silver. 1941 isn't a rare year for British silver threepence, however, mint state examples are desirable and can bring a sizable premium over silver scrap.
They aren't rare, they are, however 90% silver and worth a bit over $5 each at the time of writing in silver content alone. Many, many, many, pre-1965 quarters were made but simply the fact they were made out of silver is what makes them valuable. Now, there are, of course, some rare years of silver quarters, but there are also very common dates (like 1964).
1949, 1949 s, 1950 s, 1951 s, 1952 s