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For my part, I'll mention my favourite sci-fi novel series: the "Mission Earth" series by L. Ron Hubbard. When not concocting creative science-fiction religions, the man was quite skilled at concocting creative science-fiction literature.
The series focuses around a pair of aliens who travel to Earth to prepare it for eventual invasion. One, a classic strong-jawed Good Guy-type is sent to ensure that the Earth doesn't self-destruct before the aliens are ready to take ownership. The other is a truly terrible (and enormously entertaining) person who is sent to Earth to stop the first, yet never seems able to get one up on him.
A superb combination of sci-fi concepts, heroism, villainy and comedy all mixed together with drug deals, smuggling, corrupt bastards and conspiracies. Highly recommended if anyone can get a hold of all 10 volumes.