Good question. It’s actually difficult to come up with my favorite theory because everything is so interlinked in one way or another.
I’ve always been fond of quantum theory, which actually isn’t as hard to understand as you may think. Bascially all it states is that on really small scales (neutrons, protons, electrons…) energy can be measured in chunks (or “quanta”, which is where the name comes from). In which case you can only use energy values that are multiples of those chunks because there are no values in between. Its exactly the same as getting rid of decimal numbers and just using whole numbers. If I call the smallest unit of energy “h” then I can only measure energy in different whole values of “h”:
0, h, 2h, 3h, 4h, 5h….
In physics we call this unit “h” Planck’s constant. It may not seem like much of a theory but actually this underpins all of modern chemistry and is involved in all kinds of weirdness involving teleportation, lasers and things being alive and dead at the same time. If you want to learn more about quantum theory then I suggest the book “In search of Schrodinger’s Cat” by John Gribbin – it’s quite a nice introduction to these ideas.