As a side asset, to keep along with fiat, bitcoin is definitely the best. However, at its current condition, it can, in no way, in the future, try to even remotely replace fiat. I mean, come on...bitcoin is extremely volatile, how can its popularity go beyond something that has been used for centuries traditionally, when people are not even trying to trust and use it.
Moreover, not many people are familiar with the concept and not educated enough to use it, and lack of adoption makes bitcoin's value unstable, and this cycle continues. Thus most governments aren't taking the risk to try and implement it mainstream, as if they use it as official currency, their economy's value might zero down in days along with bitcoin's if and when it falls.