The best? That's sort of a difficult question to ask. There are dealers/sites that specialize in different coins/series. If you could narrow the scope down a little it would help.
For example, if you are looking at Lincoln cents, it's hard to beat:
http://www.thepennylady.com/ http://02a7097.netsolstores.com/offline.aspx (currently on hiatus)
http://www.angeldees.com/
For Indian Head Cents I'd look first at:
http://www.indiancent.com/ http://www.thepennylady.com/
Other sites I visit regularly:
http://www.markfeldcoins.com/ http://www.jjteaparty.com/ http://www.frankcoins.com/ http://www.ldrcoins.com/
These are a few of my favorites there are dozens more, depending on what you're looking for.
Others can add their favorite sites and you'll have a list of top quality people to do business with.
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