A survey of 110 campaigners by The Right Ethos, a recruitment consultancy that specialises in finding work for campaigners, placed the environmental group ahead of Amnesty International.
Friends of the Earth, the British Union for the Abolition of Vivisection and Liberty made up the rest of the top five.
Jonathan Dearth, founder of The Right Ethos, said: "The results aren't that surprising, apart from Liberty, which is a very small organisation but is seen as an exciting place to work because of the high profile of its director."
Dearth said campaigners were more likely to be motivated by their political views than those of other sector staff. "More people are showing an interest in campaigning as a career," he added.