Stop. Always works for me.
So safe words usually exist because we want to expressly declare that "stop" isn't enough.
That could be because the partners are about to embark in a consensual-non-consent scene ... or it could be because everyone wants to be super clear about their intent at all times (a really good thing). That being said, for that reason, 'stop' - isn't the best safeword; there exists a section of the community that will interpret that (without prior discussion) as ... well - not stop.
Therefore, if the goal is to agree beforehand on a good safeword, I tend to like the traffic light system others have presented here:
Green - usually only called if asked for a "light" - all is good, keep going.
Yellow - This is getting a bit much for me - I don't necessarily want to stop, but this is about my limit - maybe switch gears/give me a rest on this topic? Pursuing this specific topic is likely a red, and I'm mostly removing my consent on this specific idea.
Red - I need to stop for now. This scene, right now, is over. Anything else in the context of this scene is without consent.
These words/etc are far enough from anything that might be called "in passion" that their meaning is hard to misunderstand, and thus, make great safewords. They are also common enough that they make a good universal symbol.