I'm not sure if it works with Siri, but my favorite google voice command is "remind me to {do something} when I'm at {some place}"
I use it for stuff I forget to buy when I'm shopping, mostly. The stuff you don't buy often, like "remind me to get deodorant when I'm at Kroger."
Also the timer-type reminders/alarms are great.
I really wish it was easier to compose notes and the like. I finally found how to reliably get a newline with the voice keyboard (by saying "period newline"...the period is required).
"Her" was pretty inspiring for all of the voice-related stuff. But I'm still weird about using voice commands in public. If everyone was, I'd get over it.