I have a select element defined as such:
I was running into this same problem earlier this evening, where I saw a select option showing up as the first in my list even though I didn't explicitly create it. I was filling a list of select options in my controller and using the same ng-options syntax mentioned above by jessedvrs (except that I was also inserting the "select an option" default option in the controller rather than marking it in the HTML like he was).
For some reason the select list would always show an additional option at index zero with a value of "?", but when I changed how I filled my default option in the controller, this issue went away. I was populating the select options by making an API call, filling them inside of a promise. I made the mistake of also populating my default "select an option" option as the first thing I did in that promise, but when I did this instead outside of the promise (prior to making the API call), the select options populated the way I waned them to.
I think jessedvrs option is one solution to the problem (setting the default option in the HTML markup), but if you prefer to populate your options in javascript instead, I would suggest to still set the default option prior to making any calls to an API or processes that may still be running while the HTML is being rendered.