And along the lines of the previous two posts, The Constitution does not allow individual states to negotiate their own treaties with other countries. We can't have North Dakota say it is willing to give up its nuclear weapons in a treaty with Russia, yet have Washington State say they won't. By pursuing a national treaty, and having the authority to negotiate on behalf of all fifty states, we have a stronger voice at the table and the likelihood of a better agreement increases. Other government are more willing to talk to us, since they only have to meet one government, and not 50 separate ones.
I think that you can have many federal powers from which to choose. Along the lines of the previous post would be the idea of being able to print money. This is a federal responsibility and it should be that. As it was under the Articles of Confederation, if states had the ability to print their own money, it would be chaotic for interstate and national commerce. Value in one state would be defined differently than another. The notion of domestic business would be completely transformed if each state had to honor different currency and the value that went along with it. In my mind, this justifies why the federal government should be in charge of making and printing all money and currency.
To me, there is no question as to which power denied to the states is most important (assuming you are talking about the United States). The Constitution says that the states may not regulate interstate commerce. This is hugely important because it helps (and has helped) the US economy.
Before the Constitution, states could and did erect tariffs and other trade barriers against each other. This made it so that various states became like separate countries in terms of trade. It meant that people from one state could not trade easily with one another.
Today, economists believe that free trade is good. We have things like NAFTA to give us access to larger markets. If the states were allowed to have trade barriers, we would not even have free trade within the US. That would weaken our economy because our companies would not be able to have such big markets -- they would only be able to sell easily within their own state.