When I'm working I often open with Eddie Fechter's "Be Honest, Where Is It?" (2CM) because the effect starts with a good hook (hustling on the streets, and "teaching" them how I do it), which in turn allows me to build some rapport and interest and talk to everyone and get them involved in what I'm doing. I also like that it allows me to move more into their personal space, but at the same time is not intrusive by asking them to do anything that feels embarrassing or asking to borrow something right off the bat when they don't even know me. My favorite part though is getting the chance to talk to everyone and get them involved and hopefully laughing before the moment of magic even hits. And then when it does, and they turn those cards over, everybody is now interested in what I do and the reaction is always a big one that comes in unison. Overall, it's a good effect with a good hook that allows me to lead a conversation, move closer into their space without being intrusive, get to know everyone's personality types a little, get everyone involved and invested, and have some fun byplay along the way.
If the people I approach already seem to be in a fun mood, entertaining themselves (and aren't looking for anyone to entertain them) I'll usually go into some more quick and visual magic--which is often Ring Thing (Garrett Thomas) followed by my rubberband routine that ends with SLR by Paul Harris. After that the door is open for one or two effects more that require a bit more focus on their part.
If I'm not working, and I'm just out doing magic, I'll open with anything that feels casual and conversational. "Hey, want to see something cool with that bottle? Yeah? Cool. Let me see that bottle cap too..."