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That's the question. If you looking to invest, then you're looking for information on firms that are traded on exchanges? Are you looking for global or just national? If national, the government typically has the better data, although they are slow in releasing it. If global, well, that's pretty useless anyway.
If you are looking to serve a market, the lists that compiled for investors can be somewhat misleading. In agriculture, for example, one of the largest firms is Cargill--they are privately owned.
Dun and Bradstreet works very hard at keeping up with businesses. I find their rates for their data usually reasonable.