Personally, I’m a fan of the flag-- it’s the simplest of the lot, and easiest to digest at-a-glance.
With that in mind, I think you’re going to run into an age-old problem with your icon selection-- only a very, very small amount of icons are so ubiquitous that they are easily recognizable by a wide variety of people. Unfortunately, there really isn’t a generally recognizable icon for the concept of milestones. Whatever you choose, you’re going to have disagreement, and labeling the icon will be a necessity. In that respect, I don’t think prolonging the debate is going to do anyone any good. You’re just going to end up in the same position no matter what you do.