I don't recall ever hearing a viewer / user complain about what the developer called the thing. I also don't remember ever reading a blog where a start-up founder obsessed over it, although I'm sure there's been one or two that we never heard of.

Therefore, I simply don't think about it and typically just use "login". It's recognizable, it simple, and users know what to do with it. Everything else is simply keeping you away from the more important parts of your project. Just one man's opinion.