What would you say is the single most important idea across all of CS?

Basic bit and byte knowledge, like some other "foundation" stuff. Basically, that everything in computers are numbers under the hood.

A bunch of things I can just think where the bit and byte stuff has helped me:

- Database structures: smaller is usually faster, and you save storage. If you know a column can only store numbers from 0 to 16, make it TINYINT UNSIGNED (one byte, 8 bits) instead of 4 bytes (32 bits). Think of x4 storage gain, think when you have hundred of thousands of rows...

- Encoding stuff: learn some hexadecimal to be able to scratch your head around the bytes of anything instead of being blind about it.

- For critical or limited storage (or making smaller packets/transfers), you can use "bit masks". Instead of using 8 bytes for 8 flags, you can use just one byte (8 bits) and just "flag" each bit of it.

Etc.

Abstraction: extract the essential for this problem in this situation and make it so easily to understand for someone with domain knowledge.

Composition: the output of a function/ program / server should be digestible by another function / program / server that doesn't know anything about the first one.

Dry. Don’t repeat yourself

The ACID test