Our first, yes, but not our only. Otherwise, dolphins and whales might be extinct soon. There's nothing preventing you from supporting both, say, Amnesty International and Sea Sheperd, or Médecins Sans Frontières and Greenpeace - or all of them. This is not a binary decision.
But first we need to find out exactly which slaves have it the worst, and free all of them. We can't go around doing good things for anyone other than the absolute worst off people. Luckily this means we can spend all our time debating who has it worst instead of doing anything to help anyone.
This is snarcky but it points out as strongly as possible what is so wrong with prioritization of good causes, it should pop out automatically every time someone tries to discuss this. And I would like to add that there are enougth amount of humans for caring about all the good causes.