I love liver. Tender lamb's liver, quickly grilled or fried, right through to pigs or ox liver which is 'strong' and I consider is all the better for being soaked overnight (at least) in either milk or yoghurt, before long slow cooking with lots of onions and other tasty things. I also like chicken and turkey liver, quickly fried.
I prefer to buy liver from a reputable local butcher who knows the provenance of the beasts.
I don't like tongue or kidney - it's the texture of both of them which I find off-putting.
Brains are OK-ish when cooked nicely, not much flavour really and a rather odd texture. Elder has a similar sort of 'pappy' texture, but firmer as it is usually pressed, and also has IMO very little flavour. Sliced very thinly, it's OK. I've not seen it for many years though, and don't know if its still obtainable or if it all goes to the petfood producers.
Heart can be very nice when cooked well - it's a very well-used muscle, so apt to be tough unless cooked by someone who knows what they are doing.