Farmed fish is terrible in terms of animal welfare and pretty bad in terms of CO₂, so best avoid that. Beef is terrible in terms of CO₂, but comparatively good in terms of animal suffering (the word “comparative” is doing a lot of heavy lifting there). So if you don’t care about animals but do care about CO₂, beef is the most important one to avoid. For those who only care about animals, farmed fish and chickens are the most important to avoid.
A good read to guide your own placement on the veganism and vegetarianism scale.