I think it might be easy to focus on any of these characteristics and say that any which one is the most important, but the way I see it, being achievable actually hinges on the other two mentioned characteristics in some way, shape or form. If an objective cannot be easily understood by others, then it simply cannot be achieved due to lack of knowledge transfer about the goal. You cannot achieve your goal without understanding clearly where you want to go and why. Likewise, if an objective is not affordable, then it is not (realistically) achievable. This may depend on how you frame achievable, but I see it as if you cannot achieve it within budget, then you have not achieved it.