in order of like from most to least:
bone marrow with sea salt, pates, heart (seared rare), trotters (hard to find but great if well made.. the definition of unctuous), crispy pig's tail, headcheese, lamb or veal kidneys, mexican style tripe soup (menudo) with lime (my Mom always made tripe soup and I was never a fan until i had a good Mexican style version), chicken livers sauteed with shallots and parsley deglazed with a splash of brandy (yum!), sweetbreads, brains (they're ok, i want to give them another shot), beef liver, other kinds of kidneys.
If you want to try kidneys, i'd recommend starting with lamb and you could try a deviled kidney recipe. Lamb kidneys are mild in flavor and I like them very much, but the deviled recipe will make them more beginner friendly.
edit: i can say almost all offal I eat it because I genuinely think it's delicious (i have eaten some that I got for free that isn't top of my list, like pork kidneys or beef liver). But then again, I'm a pretty decent cook and I know what to do with it. That makes a big, big difference. Chris Cosentino's blog "Offal Good" is a great resource. Amazing recipe for heart and he shows you how to trim it properly in a video, which is nice.