And the question is also at the same time incomplete and 2 questions in one:
Which conservation organization does the best and most work
WWF and WCS do very different work and if you include many of the smaller ones the work-field even becomes more diverse. So you could actually say that WWF does the best work in the field of lobbying while a Durrell is better at saving individual species. So except expending your list to an infinitive amount of conservation organisations you would also need to give the possibility to click multiple organisations as different organisations will be the best in their niche.
And the organisation that does the most work does not have to be the organisation that does the best work.