I wonder what you actually meant by power colour. In case of cobalt for you, it is so flattering and it makes you stand out in the best of ways. I think you need a really good excuse not to wear it more often, because you do glow in it.
So, to go with that and to answer the aspect of what I feel looks best on me, it's red.
But, for me personally, power colour would mean what makes me feel powerful/strong/energized. That is a different thing. I know there are other colours that, on top of flattering, make me feel good, so when I deliberately choose colour for some purpose the list would be: red (hear me out), purple (I'm on top of this), orchid (energy!), cobalt (calming, grounding), white (clarity), black (no nonsense).