Another way to ask the titular question is: which constructed language could you actually speak and be understood by a speaker of a natural language (who doesn't also know the constructed language), i.e., could you "get by" in a country with said constructed language?
It is probably one of the a posteriori languages, such as Occidental/Interlingue, Interlingua, Interslavic, Folksprak, Lingua Franca Nova, or some other naturalistic language. Or perhaps even Esperanto.
I consider this to be nearly the inverse of this previously asked question.
EDIT: The answer could inform a language learner which constructed language to study and provide additional motivation knowing that the language could be useful for oral communication with natural language speakers.