When I looked into conlangs over ten years ago I concluded that there were three that were particularly worth knowing.<p>1. Novial, because it is extremely regular with simple rules<p>2. Ido, because it is a simplified version of Esperanto, which itself has a non-trivial number of speakers.<p>3. Interlingua, because it is roughly a simplified modern romance language, which themselves have hundreds of millions of native speakers.