I've been reading PUA esque stuff lately and something they stress is that "the opener doesn't matter", "you can open with anything". This is in contrast to the older, cheesier, tactic based PUAs who used to focus obsessively over finding the right line to open with. This advice is meant for approaching women in bars, but I imagine it holds true for most ocassions you would want to talk to a stranger.

In general if you're in a social situation where strangers are approaching each other, then people are generally receptive to people approaching them and will be grateful that you are putting in the work of initiating contact and not them. People also understand that it's sometimes awkward to initiate with strangers, and will usually try to help you smooth things over if you initially make a rough landing. If you come in awkwardly, then you can gauge their reaction, calibrate to find a more appropriate tone, continue without drawing attention to the initial awkwardness, and things will be fine.

Personally, I think the best way to open a conversation with a stranger would just be to go up to them and say "Hey, I'm __" and offer a handshake. It's straightforward and shows confidence.

If you're in a situation where it's not necessarily common to approach strangers, you'll probably have to to come up with some "excuse" for talking to them, like "that's a cool shirt" or "do you know where the library is?". Then you have to transition that into a conversation somehow. I'm not really sure how to do that part.

EDIT: If an approach goes badly, don't take it personally. They might be having a bad day. They might be socially awkward themselves. And if someone is an asshole to you just for going up and saying hi, they are the weirdo, not you. On the other hand, if ten approaches in a row go badly, then you should take it personally.