Quick answer
Lo siento
the everyday version
Perdón
the alternative — see below for when
Which one to use
Lo siento is the apology with weight. Use it when you actually did something wrong, or to express sympathy after bad news.
Perdón is the light, everyday version: you stepped on a foot, you interrupted, you need to squeeze past. For a mistake at work or with a friend, perdona or perdone hits the middle ground.
All the versions
Lo siento.I’m sorry — a genuine apology.
Lo siento mucho.I’m so sorry — stronger, also for bad news.
Perdón.Sorry — small, everyday slips.
Perdona.Sorry — informal, to one person.
Fue culpa mía.It was my fault — taking responsibility.
Where you are matters
The wording is stable across countries; <b>disculpa</b> replaces <b>perdona</b> in much of Latin America for the same small apologies.
Check yourself
A friend tells you their grandmother died. What do you say?