Wow thank you so much everyone, and @betteirene...
wowowow thank you so much for that detailed answer! I've been trying to check a few recipes, but I essentially want to create my own recipe, or know if I do make substitutions, what effects will come out of it, because I love being a scientist in the kitchen as well! We all are essentially aren't we? haha...that's why I didn't fail chemistry.
With my chocolate cake, I wanted to use cocoa powder and a combination of oil and butter to see if I could create mini cupcake that wasn't dense but still moist.

I've been converting all of my baking recipes to coconut oil and I'm having delicious, moist (and healthy) results. It has taken time and a lot of trial and error, but has been so worth it.

Butter, bar none, makes the best cakes. The furthest I'd go in terms of substitution is margarine, and even then, it's at most half butter, half margarine in a recipe. Oil gives cakes moistness, but tends to yield a denser crumb. Shortening is best for pastry, and sometimes frostings/icings. Don't think the taste in cakes would be very nice. Good luck!

Are you following a recipe, or are you developing one? If you’re a follower, use what is called for in the recipe. Depending on the complexity of the cake, it can be risky to substitute one fat for another without losing texture or flavor.

But if you’re developing your own recipe and you’re after fluffiness, use shortening. When you cream shortening and sugar together, the shortening will hold more air bubbles than butter, so you get more lift, which is a big part of fluffiness. Also, shortening is 100% fat, while butter contains a bit of water, which lowers its melting point, which affects the cake’s tenderness (softness) and its rise. What I'm going to suggest next is an oversimplification (I'm leaving out any mention of baking powder, baking soda, liquids, and the technique of creaming), but it's an example that works: Think of a brownie--how many are more than 1" tall? None of them. How many brownie recipes call for butter? All of them. That's the basic difference between butter and shortening in cakes.

(In many of Julia Child’s recipes for pie and tart doughs, she calls for butter along with a bit of shortening to help tenderize American flour.)

But gosh, butter just tastes soooo good, so please, if you’re after delicate flavor, say, for a pound cake, don’t use Crisco, even if it’s the butter-flavored version. To get around the flatter flavor of shortening, boost the flavor profile of the cake by using pure, full-bodied extracts and spices in the cake and its frosting or glaze. Try layering the flavors, like using coffee or buttermilk as the liquid in a chocolate cake, and coffee or sour cream in the frosting; use buttermilk and lemon zest to punch up a vanilla cake; add no more than ¼ teaspoon of cinnamon to a banana cake; add ginger with the cinnamon in a carrot cake and the cream cheese icing, and so on.

But don't think that if some shortening helps make a cake fluffy, a little more will make it fluffier. Adding extra just makes the cake heavy and will give it a greasy mouth-feel. What's the proper proportion of flour-sugar-butter or shortening? I don't know because it depends on so many factors like how many eggs, are they added whole or will the whites be whipped to aid in leavening, if baking powder, baking soda, or both will be used. . .this is exactly the reason I think we should trade aprons for lab coats: We're all scientists!

Butter, absolutely. I'd be afraid lard would give the cake, especially if it's a delicately-flavored one, an "off" taste. Vegetable shortening works better for flaky type applications like crusts and biscuits, not where you want a tender crumb like a cake. I know a lot of box cake mixes use oil, but if you're using a recipe that's written for butter, I'd be afraid to sub oil, since the chemistry may be different. Following your recipe is your best bet for success.

Butter or oil!

So I wouldnt use shortening for something like Pound cake?Is shortening best for biscuits/shortbread cookies? something that creates a crumbly texture?

I once ran out out of shortening & the local store only had lard. The cakes turned out really moist. With shortening, it's a little bit drier but I don't think an average cake eater would notice.

Butter. And there are several cakes on this site that use vegetable oil (and olive oil) and are very light and moist.

Butter