My preferences..........
1. Ox Liver
2. Pig Liver
3. Lamb Liver (too powdery for my liking)
4. Anything else that has a liver...........
I put seasoned flour in a plastic bag and add flour to make sure it's thoroughly coated (a teaspoon of turmeric can be quite nice). Then fry (with onions) until just cooked. Remove liver and onions from pan, add stock (of whichever animal you have used) and heat up slowly, use some of flour from bag to thicken gravy...........add liver and onions back to gravy, heat through and serve.