What are the best intelligence agencies of the world?

Answer 1

Such a question cannot really be answered, especially given that the layman will likely go for either a large HUMINT based system (CIA type) or another analyst system that has had a great deal of success (MI6).

Therefore, it really depends on the branch of intelligence they are getting at. NSA from the United States is particularly good about gathering signals intelligence, and due to the incredible amount of funding available isn't likely to be beaten by anyone in that regard. On the other hand, first world powers are notoriously bad at conducting human intelligence, i.e. spying directly, and as such cannot really be rated.

For conducting ops, the Mossad and CIA have been incredibly effective over their history, though arguably now have lost some of their touch in recent years. MI6, however, definitely has one of the best analysis programs on the planet, particularly since they aren't as hampered by red tape as much of the rest of the world and share information among themselves. Also, let us not forget the effective programs of domestic intelligence services that many countries have, including the intelligence/justice hybrid of the FBI in America, or CSIS of Canada. But the biggest part of the picture is that the majority of these intelligence services are willing to help one another when needed (with the exception of the Mossad). Many intelligence achievements are not done by hoarding information in one agency or country, but rather from joint operations among the many, unfortunately, the greatest intelligence failures seem to come from acting alone.