Caesar cipher
Move every letter three places along the alphabet. That sufficed in 50 BC and suffices for nothing today.
The simplest substitution cipher: each letter is replaced by the one a fixed number of places further along the alphabet, wrapping around at Z. Suetonius records that Julius Caesar used a shift of three in his correspondence. The key space consists of 25 possibilities, which means the cipher can be broken by trying all of them by hand in under a minute.
It survives as a teaching example and, in disguise, as ROT13. Its real value is illustrative: it shows that a secret method is not the same thing as a secure one, and that a key space must be large enough to make exhaustive search impossible. Both insights were formalised much later, by Kerckhoffs and Shannon respectively.