Perdí mis llaves

Perdí mis llaves
 

El otro día se me perdieron las llaves.
El otro día se me perdieron mis llaves.
El otro día perdi mis llaves.
Perdí mis llaves el otro día.

Por un gringo como yo todos signifíca lo mismo. ¿Es uno mejor que los otros?

Como siempre, gracias de antemano.

Use las llaves. First person is established by the verb's ending, so it's redundant to use a possessive determiner that expresses the same person. In some places, native speakers of Spanish will double up on possession. Most, however, do not.

The "no fault 'se'" construction means that you aren't taking the blame for losing your keys.
Your last two sentences mean that you blame yourself for losing your keys. To the English-wired brain, this is normal; of course it's our fault. But the Spanish don't take blame for forgetting, losing, dropping, breaking, etc. That's why the "no fault (accidental) 'se'" construction exists.