Definitely not doña and not lo siento. I’d say “disculpe usted.”
Avoid doña completely unless you are trying to be funny. As for “lo siento”, use it when you actually feel sorry. For instance, somebody says, “my cat died”, and you respond with “lo siento.”
“Disculpe” is more like “excuse me”, so it works when you are trying to attract attention. If you want to be more casual, you can simply say “oiga”.
For German speakers, it’s like the difference between “es tut mir leid” and “entschuldigung”. Would you say “es tut mir leid, Frau” if somebody drops something?
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